Vegetarian dish help

This dish is not tomato based but it is really good. https://food52.com/recipes/24210-creamy-polenta-with-sauteed-apples-mushrooms-calvados

My kids--who are passionate meat-eaters-- love roasted vegetables over polenta for dinner. Roast a mix of vegetables, being careful to roast similarly textured vegetables together, i.e.: start with a diced potato, carrot, diced sweet potato. When they're looking good at softer vegetables--broccoli, onions, summer squash--and then finally cherry tomatoes. You just want everything to be nicely roasted at the same time. Very easy. Sprinkle with gremolata. Your vegetarian friend will be grateful.

Eggplant, zucchini, onions, green pepper, a Ratatouille

As you're making a ragu , I'd consider vaguely mimicking it with a Peperonata. There's many recipes out there, but my preference is to char the skin off of the bell peppers and stew them with onion, roast garlic and a little fennel. Finish with olive oil and basil. Would go well on polenta, be a vegetable ragu that could shake ahnds comfortably with the short rib ragu and if wanted, would take an egg like a champ.

As like Nancy says, beans are a great idea, canollini (white) beans keep it very Italian, but other beans are very versatile. shakshuka can be adapted into the flavors of any cuisine by the flavoring of the sauce it's in, and it has variants that a native to many cuisines; in European cooking it's often called eggs in purgatory; a good tomato basil sauce can keep the flavors Italian and goes great with eggs. You can also do a ball of Buffalo mozzarella or a block of feta or other fairly solid cheese stewed in a tomato sauce. Or some veggie balls, you can be them fully made from many grocery stores in the frozen section, or make them yourself made from mashed lentils or from a meat-substitute like tempe.

If you're not stuck on it being in a tomato sauce, many mushroom stews are delicious and would complement a good polenta

You could use this delicious mushroom bourguignon as an inspiration: https://smittenkitchen.com/2009/01/mushroom-bourguignon/
It's a good match for polenta.

If vegan, some high protein beans (pinto, red, etc) in the sauce.
Or seitan.
If lacto ovo vegetarian, make shakshuka with eggs or a layered cheese tomato sauce.