Lay onion and mushrooms single layer on parchment-lined baking sheet and bake 15 minutes, or until tender. Remove from oven, set aside to cool.
Boil pasta according to package directions. Drain and toss with oil; set aside.
Heat butter in large skillet on medium-high heat; add onions and mushrooms, and cook 2 minutes, stirring often. Reduce heat to medium; add spinach and olives, and cook 2 minutes. Stir in pasta.