Preheat the oven to 350F and line a baking sheet with parchment paper. In a large mixing bowl, combine the diced onion, zucchini, bell pepper, olive oil and fresh rosemary, and season with salt and pepper to taste. (I used 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 (as pictured above) to give them a flat bottom to them stand up straight-- you don't want them to fall over with the filling inside! Spoon the filling into the center of each squash and bake at 350F until the squash is fork-tender, about one hour. Serve warm.
If you'd like to spice up these squashes a bit, feel free to add your favorite dressing or sauce. I added a drizzle of Creamy Caesar Dressing (from my cookbook) to these, and they were unbelievably delicious!