Heat olive oil in a Dutch oven or large stock pot, sauté onion for 4-5 minutes, then add garlic and zucchini and sauté for an additional 4-5 minutes. They should still be crisp tender.

Add remaining filling ingredients, stir to combine. The juice from the diced tomatoes should be enough simmering liquid, if not add a little water. Cover and simmer for 10 minutes. Use a slotted spoon to transfer veggie mixture to a casserole dish.

In a 3 quart saucepan bring water and salt to boil. Slowly whisk in grits and cook over low heat, stirring often until the cornmeal is tender, about 15 minutes. Stir in butter to finish. Spoon polenta over veggie mixture, then use a rubber spatula to spread almost to the edges.

Preheat oven to 350 degrees. Place casserole on a shallow baking sheet and bake for 30 minutes. Can finish under broiler for a golden top. Allow to rest 10 minutes before serving.