Guru Nanak wrote a passage of very lengthy bani where he clearly and precisely ridiculed any connection between vegetarianism and religion.The bani in question appears on page 1289 to 1290 of the Guru Granth Sahib and is reported to have been written by the Guru when he ate meat at a hindu festival.

The Bani

Pahilan masuhn nimmiya masai andari vasu (Guru Nanak, Pg1289-1290 Guru Granth Sahib, Raga Malar)

Born out of flesh, in flesh does man live.
With life comes love of flesh, bones, skin, limbs
All are flesh.
As from flesh the living body issues, the breast that is flesh,
It grasps in mouth.
Of flesh is the mouth, of flesh the tongue,
Through flesh is breath drawn.
As is man grown up, matrimony he enters,
And brings home flesh.
Of flesh is born flesh,
With flesh are all relationships established.
By the touch of the holy Perceptor is the Divine Ordinance realized,
Whereby comes fulfilment.
Not by man's own effort comes liberation:
Saith Nanak: Such talk only to perdition leads. (1)

Sloka Guru Nanak
Over eating meat dispute blind fools,
Of enlightenment and contemplation ignorant.
Which is flesh, which vegetation?
Which to sin leads?
After the tradition of the gods, was the rhinoceros slaughtered,
And of its flesh burnt sacrifice offered.
Those that at night-time gobble up flesh,
Pose to discard flesh and stop their noses.
A great show of piety they make before others --
Of enlightenment and contemplation ignorant.
Saith Nanak: What avail arguing with the purblind.
Neither would they say anything sensible,
Nor follow whatever is told to them.
Truly blind is one whose actions are blind --
The mind's eye he lacks
From mother's and father's blood are they born,
Yet flesh and flesh they abjure.
As at night come woman and man together,
Foul is their conduct.
Of flesh are we begotten, from flesh born;
Of flesh are we vessels.
Brahmin! ignorant of enlightenment and contemplation,
Thou art yet reputed to be wise.
Revered sir! the flesh that is brought from outside dost thou discard,
But those in thy home, of flesh made, thou thinkst good.
From flesh are all living objects made.
In flesh is life lodged.
Those by the false perceptor instructed;
Consume what is forbidden and discard what is lawful to each;
Know, that of flesh are we begotten, from flesh born;
Of flesh are we vessels.
Brahmin! ignorant of enlightenment and contemplation,
Thou art reputed to be wise.
In Puranas and in Muslim books is consuming flesh commended;
In all Four Ages is flesh in general use.
In sacrifices and in fine wedding ceremonials
Is use of flesh in vogue.
Women and men, of flesh are born,
As also are kings and great potentates.
Shouldst thou feel to hell are they wending their way,
Receive thou not their charity.
What absurdity that the donor to hell may go,
While the recepient to enter heaven is believed.
Brahmin! ignorant thyself, others thou dost instruct --
Strange that thou art declared wise!
Brahmin! thou knowest not where from is flesh born:
From water grows grain, sugarcane and cotton;
From water is the world reckoned born.
Water claims to be pure, yet from water comes much that arouses evil.
Saith Nanak after contemplation:
Shouldst thou discard all such delectable objects,
Mayst thou claim to have exercised true renunciation. (2)

Commentary

In the first stanza Guru Nanak establishes the fact that we are a creation of flesh, being composed of flesh and being born out of the flesh of our parents.

At the beginning of the second stanza Guru Nanak decries the duality of vegetarians who expound against eating meat. Guru Nanak says that these people have their priorities mixed up, they fight about the virtues of not eating meat, yet they do not instead spend their time contemplating God. He says that everything is a living thing, whether one is killing an animal or a plant, he says that can the vegetarian please tell him where the sin lies? If he is killing living things whether they are plants or animals. Then Guru Nanak follows this right away talking about how animal sacrifice (rhino) was such an integral part of Hindu worship, one is willing to sacrifice animals for religion, yet say do not eat flesh for religion, another contradiction exposed by the Guru. Guru Nanak then talks about the duality of these people who commit countless sins and then make a public show of their "great piety" by saying that they don't eat meat because it is a sin, another contradiction. Guru Nanak then talks about how the Brahmin vegetarian is a creation of flesh, yet he holds the flesh from outside as being sinful to eat, Guru Nanak is saying that aren't you Brahmin vegetarian a creation of flesh as well as your family also something to be abhorred if you are following your own criteria that flesh is sinful? Guru Nanak talks about these false preceptors who are wrongfully telling people to consume what should not be, and telling people to not eat what should be eaten. Guru Nanak then establishes the fact that eating meat was a historical fact of both the Muslim and Hindu faiths and it has been a part of daily life for many ages, yet why is the Brahmin vegetarian denying his own history? Then Guru Nanak talks about the absurdity that the kings who eat meat should all go to hell, yet the Brahmin vegetarian who lives off their donations and charity shall go to heaven. Finally Guru Nanak concludes by saying that all vegetables and meat are the same because their basic ingredient is water and that the Brahmin vegetarian if he really wants to practice his religious renunciation instead of not eating meat he should give up water altogether.