Heat 3 Tbsp. oil in a large skillet over medium-high. Add mushrooms and thyme, season with salt and pepper, and cook, tossing often, until mushrooms are golden brown and tender, 10–12 minutes. Transfer to a large bowl.
Increase heat to high and heat 2 Tbsp. oil in same skillet. Cook beef, tossing often, until browned on all sides, 5–8 minutes. Transfer to bowl with mushroom mixture.
Reduce heat to medium-high, add remaining 2 Tbsp. oil to skillet, and cook onion and garlic, stirring occasionally, until golden and softened, 8–10 minutes. Add broth and cream to pan. Return mushrooms and beef to pan and bring to a simmer, and cook until reduced by half, 20–25 minutes. Whisk in sour cream, Worcestershire sauce, and mustard; season with salt and pepper. Simmer until sauce is thick and creamy, 8–10 minutes.
Meanwhile, cook egg noodles in a large pot of boiling salted water, stirring occasionally, until al dente. Drain and transfer to a large bowl. Add butter and a large pinch of parsley and a large pinch of chives; toss until butter is melted.
Arrange noodles on a large platter and spoon stroganoff over top. Garnish with additional parsley and chives.
Note: I frequently serve this dish with steamed rice instead of the noodles because it lightens the dish a bit.