Bring a large pot of salted water to a boil. Cook the pappardelle for approximately 8 minutes, or according to the package directions, until just under al dente. Reserve ½ cup of the pasta cooking water, then drain.
Melt the butter in a large Dutch oven over medium heat. Add the onion and mushrooms. Sauté until the moisture released by the mushrooms has evaporated and the vegetables are nicely browned.
Add the minced garlic and sauté briefly until fragrant, taking care not to let it burn.
Pour in the chicken broth and stir, scraping up any browned bits from the bottom of the pan.
Reduce the heat to medium-low and stir in the heavy cream. Simmer uncovered for 10-15 minutes, stirring occasionally, until the sauce has thickened.
Season the sauce with black pepper and sea salt to taste.
Add the drained pappardelle to the sauce and toss gently to coat. Cook for 2-3 minutes, allowing the pasta to finish cooking and absorb some of the sauce. If needed, add the reserved pasta water 2 tablespoons at a time until the sauce reaches your desired consistency.
Stir in freshly grated Parmesan cheese to taste, then remove from the heat.
Divide between serving bowls, add extra Parmesan if desired, and serve and enjoy!