Preheat oven to 425 degrees. Grease a 2-quart or 9x13" casserole dish with olive oil.
Scrub all the vegetables and dry well. Dice the potatoes, onion, squash, and peppers into 1-inch cubes. Slice the tomatoes into ½-inch thick slices.
In a large mixing bowl, toss the potatoes and onions with 1 ½ tablespoons of oil, ½ tablespoon minced garlic, 1 teaspoon salt, ½ teaspoon oregano, pepper, and crushed red pepper flakes if using.
Spread out the root vegetables into the prepared pan and bake for 30 minutes.
Meanwhile, toss the squash and peppers (and/or any other tender vegetables) with the remaining ½ tablespoon oil, ½ teaspoon salt, ½ teaspoon oregano, and ½ tablespoon of garlic.
Remove the casserole from oven and spread the squash and peppers over the root vegetables. Top with spaghetti sauce and sliced tomatoes and return to the oven to bake for 20-22 minutes, or until the potatoes are fork tender.
Serve warm with fresh Parmesan cheese if desired.
Vegetables: You need about 8 cups total of vegetables for this casserole. Use any combination of potatoes, sweet potatoes, carrots, onions, zucchini, squash, mushrooms, pepper you desire. Keep in mind that root vegetables will need a longer roasting time, and softer vegetables, like peppers or summer squash, will need less time. Potatoes/Onions: Any variety of potatoes or onions can be used for this casserole. Tomatoes/Tomato Sauce: Feel free to simply use sliced tomatoes OR simply use the spaghetti sauce or omit both the tomatoes and sauce and the dish will still be delicious. Leftover vegetables can be stored in the refrigerator for up to 5 days.