Preheat the oven to 350F and line a baking sheet with parchment paper. In a large mixing bowl, combine the diced onion, zucchini, bell peppers, olive oil and fresh rosemary, and season with salt and pepper to taste. (about 1/4 teaspoon each.)
Arrange the squash halves cut-side-up on the baking sheet. If they are wobbly, you can slice the rounded bottoms off to give them a flat bottom to them stand up straight-- you don't want them to fall over with the filling inside! (You can microwave the squash for 10 minutes to make cutting in half easier). Spoon the filling into the center of each squash and bake at 350F until the squash is fork-tender, about one hour.
If you'd like to spice up these squashes a bit, feel free to add your favorite dressing or sauce, like Caesar dressing