totally agree that the vegetarian dish needs to be available for everyone since the carnivores will consider it a side dish.
Question - are you serving this or is it self serve?
One possible way that you can make less of the "vegetarian" option and not have the problem that everyone stated - running out because the carnivores took a portion first is to make the same dish but one with meat and one without - even then I would still have at least one to half ratio meat to veg
meat lasagne one pan
vegetarian lasagne (same as meat but w/out meat) half pan
we recently did this on a budget event that we served
we had a cold tuna/salmon niscoise potato salad with veggies and olives
made the base without the fish and divided the base salad into 2/3 to 1./3 ratio
and verbally offered " we have a nicoise potato salad - would you like it with fish or vegetarian)
out of 40 - about 4 went for the vegetarian option and I ended up giving the second scoop of vegetarian to the fish eaters as we got lower on the fish portion but had PLENTY of vegetarian that way. Many asked can I have some of both - which in the beginning of the line I replied - they are the same - only one has fish added, the other is the vegetarian options "oh I'll have the fish option then" LOL
Here are some complimentary suggestions that you could make as the main course either way
White Beans, Kale and Rice - complete protein for vegetarians
add chicken or mini meatballs to that as the carnivore option
Tortellini with Pesto Lasagne (holds better and might be easier than making lasagne) with Chicken Sausage added for Meat option
Baked Ziti - add meat sauce for carnivore option
Eggplant Parmesan Bake - add either meat
sauce or chicken sausage for carnivore option
a variations of this is the zucchini rollup
egg batter long thick planks of green or yellow zucchini (can be done a few days ahead0
basically just egg and flour and sauteed until cooked and golden on both sides.
Once cooled stuff with anything that hits your fancy
we like a rondele garlic cream cheese, sundried tomato and basil pesto, roasted red peppers, chiffonade of fresh spinach
can add proscuitto, pancetta or dice of genoa salami to the mix
if you are planning to heat up (works well too but great at room temp ) then add some shredded cheese of choice - provolone, mozzarella, jack, young asiago to the rondele before spreading - this will help bind everything as it heats.
you could also do the eggplant parmesan as an eggplant rollatini
Another popular and somewhat Italian fun dish that could be made two ways
Sauteed Brussel Sprouts with Pancetta and Grapes - I made a veggie spinoff of this recently when I el was riffling through my frig.
Julienned and Sauteed the brussel sprout leaves garlic evoo
added a few plops of good san marzano tomato sauce to the pan but did not stir
added a few slices of fresh buffalo mozzarella (yes, happened to have this in the frig )
and covered the pan just to melt the cheese
grated asiago over the whole thing and ate - delicious !