Vegetarian Lasagne

Why wouldn't you use what is used in a "normal" lasagna recipe? That would be vegetarian.

Or do you mean you're looking for a lower calorie lasagna recipe?

Yeah, I actually have an amazing one!! This recipe is delicious, I've made it many times (including for friends who also loved it): https://www.bbcgoodfood.com/recipes/layered-aubergine-lentil-bake

I don't generally do recipes; I just make stuff.

If you want the classic tomato sauce lasagna, then yes, reduced fat mozzarella and ricotta should work. Personally, I'd go with lowfat/2% rather than non-fat, because I've found most non-fat types to be . . . uck. Another thing you can try for more protein (not necessarily lower calories, but more protein per calorie) would be to get some soft tofu, and mix that well (puree) with the ricotta before using as filling. If you eat eggs, putting an egg in that mixture would maybe help it set up a bit better, if that matters to you.

A bit of parmesan (real not the powder stuff) might be fine, on top: It's surprisingly calorie-efficient for the protein content (note: I said "surprisingly" not "extremely" ), and a small amount when grated covers a lot of lasagna-top and is flavorful. I'd note that most parmesan uses animal rennet, if that matters to you.

Tempeh might be a good sub for the mince, if you want one; or as was suggested, lentils.

If you're willing to range beyond tomato sauce lasagna, something that's good IMO is a white(-ish) lasagna with mashed cooked Winter squash and/or pureed white beans (or chickpeas) included in the ricotta mix, chopped fresh sage alongside fresh garlic/onions as the core seasoning, and - if you like - some vieggies in the layers, really whatever you like: Peas, beans, yellow squash (holds up better than zucchini but that's also fine as long as you're OK with it getting soft), eggplant, etc. Again, could add the egg to the pureed stuff for more structural integrity, though I've found beans or a drier squash (like butternut, as opposed to, say, acorn) to hold up fine in reasonable proportions.

Other than trying to avoid the filling layers getting too runny, grainy, or breaking down, there really aren't structural ingredients to lasagna, so it's fun to experiment with different fillings and layers, IMO. (I always think of dishes as being composed of structural ingredients, that you can't mess with too freely or the end result is a different food , vs. other ingredients that can be experimented with, as long as major things like wet/dry are accounted for. YMMV.)

I use veggies (courgette, pepper, mushrooms maybe some lentils if you're looking for added protein) instead of mince them make my own 'cheese' sauce using nutritional yeast and oat cream mixed with some plant milk (much lower cal than animal milk) -but I dont eat any animals product so easy for me to say. Also, for extra low cal use layers of aubergine instead of pasta. Super healthy and super low cal!