Boil a large pot of water. Add a generous pinch of salt and cook the tortellini just to al dente, according to package instructions. Drain and set aside.
18 ounces fresh tortellini
While the pasta cooks, crisp bacon in a large skillet over medium-high heat. When it's as crisp as you like, remove to a plate and set aside. (See note.) Wipe out a small amount, but not all, of the rendered bacon fat.
3-4 slices bacon
Add 1 Tablespoon of the butter to the skillet. When it melts, add the mushrooms. Sprinkle with about 1/2 teaspoon each salt and pepper, then sauté for about 5 minutes, stirring occasionally, until the mushrooms release their liquid and are nicely browned. Add garlic and thyme. Cook for another 30-60 seconds, just until fragrant.
3 Tablespoons butter, 12 ounces sliced mushrooms, 2-3 cloves garlic, 1-2 teaspoons fresh thyme leaves, kosher salt and black pepper
Reduce heat to medium-low. Cut the remaining 2 Tablespoons butter into pieces and add to the skillet. After they melt, stir in the drained tortellini and Parmesan. Crumble or chop the bacon into small pieces and sprinkle those on top.
1-2 teaspoons fresh thyme leaves, 1/2 cup freshly-grated Parmesan cheese
Season with more salt, pepper, and thyme to taste and serve right away.