Cook pasta as package directs. Meanwhile, cook bacon in a large, deep nonstick skillet over medium heat until browned. Remove bacon to paper towels to drain. Remove all but 2 Tbsp fat from skillet. Add onion and mushrooms to skillet; sauté 5 minutes until onion is golden. Add garlic and red pepper flakes; cook 30 seconds until fragrant. Add spinach; cover and cook, stirring occasionally, 2 minutes until wilted. Drain pasta, reserving 1 cup pasta water. Add pasta to skillet with some of the reserved pasta water; toss. Add more water as necessary. Crumble bacon on top of pasta and serve with Parmesan.