Bring a large pot of heavily salted water to a boil and cook the pasta until 1 minute before al dente according to package instructions.
While pasta cooks, heat olive oil and butter in a large wide skillet over medium-high heat until the butter foam subsides.
Add the sliced mushrooms in a single layer and sear without stirring for 4 minutes until deeply browned, then stir and cook for another 2 minutes.
Add the diced shallots and sauté for 2 minutes until translucent, then add the garlic and thyme, cooking for 60 seconds until fragrant.
Before draining the pasta, carefully ladle out and reserve 1 cup of the starchy cooking water.
Drain the pasta and immediately add it to the skillet with the mushrooms over medium-low heat.
Add the ricotta cheese, Parmesan, and 1/2 cup of the reserved pasta water to the skillet.
Toss the pasta vigorously with tongs for 1-2 minutes until the ricotta and water emulsify into a creamy sauce that clings to the noodles.
Stir in the lemon juice, lemon zest, and fresh parsley, then remove from heat.
Adjust seasoning with salt and a generous amount of cracked black pepper, adding more pasta water if the sauce requires further thinning.