In a large pot of boiling salted water, cook pasta until al dente. Reserve 3/4 cup pasta water; drain pasta and return to pot.
Meanwhile, in a large skillet, melt 1 tablespoon butter over medium. Add onion and cook until softened, about 4 minutes. Add wine and cook until almost evaporated, 4 to 6 minutes. Add mushrooms; season with salt and pepper and cook until browned, about 8 minutes. Remove skillet from heat and stir in 1 tablespoon butter.
Add mushroom mixture, ricotta, and lemon juice to pasta; stir to combine, adding enough pasta water to create a thin sauce. Serve topped with Parmesan.
For even more flavor, experiment with dried porcini mushrooms.