Whisk together 1 ¾ cups of chicken broth and Dijon mustard. Set aside. Whisk together the sour cream and remaining ¼ cup of chicken broth. Set aside. (See Tip 3)
Heat butter over medium heat in a large skillet. Add onions; cook stirring occasionally until onions become tender, about 10 minutes. Do not brown.
Add mushrooms. Increase the heat to medium high; cook stirring occasionally until all of the liquid has evaporated and the mushrooms have browned, about 4 to 5 minutes. Reduce the heat to medium and add the garlic, salt, thyme and pepper; cook for 30 seconds or until fragrant.
Add flour; stir to combine. Cook for a minute or two until the flour has browned.
Add chicken broth/Dijon mustard mixture. Cook, stirring constantly until mixture thickens and is bubbly, about 4 minutes.
Add sour cream/chicken broth mixture to mushroom mixture. Cook, stirring constantly until mixture well combined.
Remove from heat; add Parmesan cheese. Stir until cheese is melted. Correct the seasonings. Garnish with minced fresh parsley if desired.
Serve over egg noodles or the pasta of your choice. Yield: 6 servings.