Bring a large pot of salted water to a rolling boil. Add the pasta and cook until al dente according to the package directions. Reserve 0.5 cup of pasta water before draining.

While the pasta cooks, heat the olive oil and 1 tablespoon of butter in a large skillet over medium-high heat until the butter foams.

Add the onion and mushrooms to the skillet. Cook, stirring occasionally, for 8 to 10 minutes until the mushrooms release their liquid, it evaporates, and the edges turn golden brown.

Stir in the remaining 2 tablespoons of butter, the garlic, and the Dijon mustard. Cook for about 1 minute until the garlic is fragrant.

Pour in the broth, lemon juice, and lemon zest. Let the liquid bubble for about 1 minute until slightly reduced.

Remove the skillet from the heat. Add the drained pasta, Parmesan, and a splash of the reserved pasta water. Toss until the cheese melts into a glossy sauce, adding more pasta water if needed.

Sprinkle with parsley and serve immediately.