Preheat oven to 425. Add diced onions, 1/2" sliced zucchini and squash to a bowl and mix. Top with oil, garlic and Italian seasoning and toss to evenly coat well.
Spread vegetables onto a baking sheet in a single layer. Roast for 10 minutes. Flip vegetables over with a spatula and return to the oven for 10 minutes more, or until they are as tender as you'd like.