Meanwhile, cut bacon into 1/2-inch pieces. Put bacon and onion into a 10-inch nonstick skillet. Cook over medium heat until bacon is golden and a little crispy, 3 to 4 minutes. Stir in garlic and remove from heat.
Meanwhile, put Parmesan, mascarpone, ricotta, salt, pepper to taste and crushed red pepper in a large bowl. Use a rubber scraper or wooden spoon to thoroughly blend the mixture.
Add pasta to the boiling water. Stir well and boil uncovered until al dente (nearly tender), usually 10 to 12 minutes. Set a timer and then start tasting the pasta near the allotted time. Al dente pasta has a toothsome texture (not at all soft and mushy) and tastes fully cooked. Scoop out and reserve about 1 cup of the pasta cooking water.
When pasta is done, drain it in a colander and then add it to the cheese mixture. Top with the bacon mixture. Use a large spoon to stir to coat all the pasta with the cheese mixture. Add a little of the reserved pasta water if needed to loosen the mixture.
Serve immediately, garnished with sprigs of thyme. Pass extra Parmesan, if you wish.
Variations: Instead of mascarpone, use 6 ounces cream cheese at room temperature blended with 1/4 cup half-and-half.