Best-Ever Beef Stroganoff

Cook egg noodles: Prepare egg noodles according to package instructions and toss with butter while still hot. Cook beef: In a large skillet over medium-high heat, heat vegetable oil. Working in batches if necessary, add beef and sear 8 to 10 minutes, until golden on all sides, and season with salt and pepper. Remove beef and place in a large bowl. Lower heat to medium and add one tablespoon olive oil. Add mushrooms and cook until golden and tender, 8 to 10 minutes. Remove from skillet and place in bowl with beef. Add remaining olive oil and add onion. Cook until tender, 6 minutes. Add garlic, thyme, and tomato paste and cook until fragrant, 2 minutes more. Stir in broth, mustard, and Worcestershire sauce and bring to a simmer. Make cornstarch slurry : In a small bowl, whisk some of the hot broth mixture into your cornstarch.  Pour cornstarch slurry into skillet and continue to simmer until thickened by about half, 10 to 12 minutes. Remove from heat and stir in mushrooms, beef, and sour cream. Season with salt and pepper. Spoon beef mixture over prepared egg noodles and garnish with a dollop of sour cream and parsley before serving.