Nearly all of the alliums are naturally perennials in the first place. We take advantage of the outsized stems and bulbs of some in their juvenile stages, such as leeks and onions. Most form some sort of bulb, that is used depending on the hardiness such as garlic and some of the leeks. If Lancelot is hardy for you, you should definitely be able to treat it like a perennial. You will almost certainly not get as thick stems as however. More on the order of an oversized scallion. How long the clumps last before running out would be another question. You might be able to treat it sort of like a non-bulbing multiplier.

Last year was a very mild winter on a whole, though we did have one weekend with -15F temps which would put us in a zone 5. I am not confident it would survive a standard long winter.

I thought leeks and onions were biennials, lasting 2 years?

So maybe I should harvest just a few stems? Should I let it flower? The scapes are tasty.

Actually leeks and onions are perennials we just tend to grow them for the first years growth patterns. They tend to be a little frost tender for zones 4 and 5 and some allium species and varieties are even more tender than that. The multiplying onions are indefinite perennials obviously. The seed/bulb part of that species has a bit different growth habit but still settles in just that for the most part those varieties are less frost hardy and display distinct day length preferences.

Chances are Lancelot is not dependably hardy but each variety has some variation. It is possible that you have a hardier specimen than the average. But chances are not. You can of course provide protection with a good mulch in the fall through early spring. That might be all that is needed for the Lancelot to make it through your winters. I would not count on it at all here in Minneapolis. Go ahead and eat the scapes, flowering and setting seed or bulbils weakens the root which you would need to strengthen to continue into future years. The original leek parent species was definitely a perennial; so if your space is not too harsh your Lancelot might well revert.

Note that it is very easy to falsely label leeks and onions as biennials, since two years is about the maximum we use of their life cycles, but 2 years is just the start as it is with all the multipliers, and also elephant and regular garlic.

Thanks... I will harvest the scapes and see what happens next year.