I would trim a bunch of onions, yellow and green squash, eggplant, red peppers, etc. into half inch cubes, then toss them in olive oil, salt and pepper, and roast them on a cookie sheet...400 for half hour.
While that is working, start a red sauce...canned crushed tomatoes, a touch of sugar, crushed red pepper, veggie stock, let it reduce and simmer in your soup pot.
When the veg are done put them in the sauce.
Then make a pot of spaghetti noodles to serve as the bed for your sauce.
My advice honestly, is not to try too hard to convince them or anything. I don't know if that is your plan. Also don't use a bunch of fake meats for them to compare to their idea of what it should taste like. Do simple vegetable and grain dishes that stand alone nutritionally and have so much flavor that none of you miss the meat.