Bring salted pot of water to a boil while prepping ingredients.
Melt butter in 12" skillet set to medium heat and add onions. Saute until onions begin to tender, add mushrooms and continue to saute until mushrooms and onions are tender. Add minced garlic and cook 1-2 minutes longer, or until fragrant.
Add chicken broth to the skillet gradually, scraping the bottom of the pan incorporate any burnt on bits into the liquid. Turn the heat up to medium high to bring liquid to a boil, then reduce to a simmer.
Add heavy cream, half of blue cheese crumbles, Parmesan, and thyme. Stir to combine. Let simmer until liquid thickens to a sauce consistency. ~15 minutes.
While sauce is thickening, cook penne pasta according to package instructions.
Once sauce reaches desired thickness, add fresh ground pepper and salt to taste.
Drain pasta from water and add to the blue cheese sauce along with parsley. Toss to combine.
Plate pasta and sprinkle with remaining blue cheese and serve.