Cook pasta shells in boiling salted water according to package directions until al dente; drain and set aside.
Melt butter in a large skillet over medium-high heat. Add sliced mushrooms and diced shallots; cook for 5-7 minutes until mushrooms are golden brown and liquid has evaporated. Season with salt and pepper.
Add minced garlic and fresh thyme; cook for 1 minute until fragrant.
Whisk in flour and cook for 1 minute until lightly browned.
Gradually add beef stock and Dijon mustard, whisking constantly. Bring to a boil, then reduce heat and simmer for 4-5 minutes until sauce has thickened.
Stir in cooked pasta and sour cream; heat for 1-2 minutes. Add Parmesan cheese and let melt for 1 minute. Stir in fresh parsley and adjust seasoning with salt and pepper.
Serve immediately while hot.