Rinse and peel onion. Rinse squash, mushrooms, and cauliflower. Chop vegetables into bite-size pieces, all equal size.
In a medium bowl, add oil, salt, pepper, and cut veggies. Stir until veggies are well coated.
Spray a baking sheet with non-stick cooking spray. Spread veggies on the baking sheet in a single layer. Bake until veggies are browned at the edges, about 30–35 minutes.
During last 10 minutes of baking time, cook pasta following package directions. Reserve ½ cup of pasta water. Drain pasta.
In a large skillet, add veggies and drained pasta. Heat over medium heat for 2–3 minutes. Stir frequently. Add some of the reserved pasta water to moisten and make a sauce.
Transfer mixture to a serving bowl. If using grated cheese and fresh herbs, add now. Serve hot.