In a large pot of boiling salted water, cook pasta according to package instructions. Reserve 1/4 cup pasta water; drain pasta and return to pot.
Meanwhile, in a large skillet, heat half the oil over medium-high heat. Add mushrooms and season with salt. Cook, stirring occasionally, until browned, about 6 minutes. Add mushrooms to pasta. Reduce skillet heat to medium and add remaining oil, brussels sprouts, and garlic and season with salt and pepper. Cook, stirring often, until brussels sprouts are soft, 4 to 6 minutes.
Add brussels sprouts mixture and lemon zest to pasta. Stir in enough reserved water to create a thin sauce that coats pasta. Add lemon juice; season to taste with salt and pepper.