You can put the cheese on top before you cook it. When it goes golden and bubbly, your lasagne is done.
For a vegetable lasagne I usually use about 200g mushrooms (sliced), one onion (chopped), two carrots (roughly grated) and a red pepper (deseeded and chopped). Start the onion off in a deep frying pan with a liitle oil first. When it begins to soften add the rest of the veg and the tomato sauce.
To go with it a bit of green salad and some garlic bread is usually sufficient. If you're feeding lots of people though you might need some chunky chips or potato wedges to make it go further.
For dessert I'd avoid anything too stodgy as the lasagne will be pretty filling. Ice cream with fresh berries and a squirt of raspberry sauce? Nice and easy, and refreshing to end the meal. I recommend either that or some sort of fruit pavlova.