Set out sour cream to allow it to come to room temperature.
Cook noodles according to package directions. Reserve 1 cup of the pasta water. Drain noodles and set aside.
In a small jar, combine half & half with the Dijon mustard, Worcestershire sauce and flour. Screw on lid and shake to combine, until there are no lumps. Set aside.
Divide mushrooms and finely chop about 1 cup of the mushrooms. Set aside.
Heat butter in a large skillet over medium heat.
Add the diced onion and season with salt and pepper as the onions cook. Sauté for 3-4 minutes until translucent, stirring frequently.
Add the sliced mushrooms to the pan and cook until they release their liquid and begin to brown along the edges, about 7-8 minutes.
Add the finely chopped mushrooms and continue to cook for another 5-6 minutes. Sauté until all the juices have cooked off.
Add minced garlic, mushroom seasoning, and thyme to the pan and cook until fragrant about 1 minute.
Add the reserved flour/mustard/cream sauce to the skillet. Cook, stirring often until sauce thickens. Scrape up any browned bits in the bottom of the pan.
Add reserved pasta water a little at a time until sauce is the thickness you like. Reduce heat and simmer until sauce is creamy and smooth, about 2-3 minutes.
Remove skillet from heat. Add the sour cream to the sauce in the skillet. Stir together until smooth and creamy.
Stir in cooked pasta until coated with the sauce.
Serve topped with a sprinkle of chopped parsley.
Notes
TIP: If sauce gets too thick as it cooks, thin it out by adding a little more water to the sauce.