Boil Pappardelle according to package directions in salted water. Drain and set aside.
Add olive oil and 2 tablespoons butter to a large skillet over medium heat.
Once the pan is hot and butter is melted, add diced onion and sliced mushrooms to the pan.
Sauté, stirring frequently until mushrooms release their liquid and cooks off. Continue to cook until mushrooms are lightly seared and browned and onions are soft and translucent, about 8-10 minutes.
Add another 2 tablespoons butter to the pan along with the minced garlic and Dijon mustard. Stir to combine with mushrooms and onions in the pan, about 1 minute.
Add chicken broth and lemon juice to the pan and simmer for 2-3 minutes until sauce is bubbly and reduces by about half.
Turn off heat and stir in grated Parmesan cheese and chopped parsley until cheese melts into the sauce.
Stir in up to a cup of hot pasta water to the pan to thin out sauce to obtain desired consistency.
Taste sauce and season with salt and freshly cracked black pepper.
Toss the sauce with cooked Pappardelle noodles.
Serve immediately and top each serving with additional freshly cracked black pepper and a sprinkle of fresh chopped parsley.