Heat a large deep pan or Dutch oven over medium-high heat, add 2 Tbsp oil, then sear the beef in 2 batches for 1 minute per side in a single layer without moving it around too soon. The strips should pick up dark brown edges and lose their red color quickly, which means they are searing instead of steaming.

Transfer the beef to a plate and cover it to keep warm, then add 2 Tbsp butter, the chopped onion, and the sliced mushrooms to the same pan. Cook for 6-8 minutes until the mushrooms release their moisture, the pan looks drier again, and everything turns soft and lightly browned.

Stir in 1 minced garlic clove and cook for 1 minute, then sprinkle in 1 Tbsp flour and keep stirring for another minute. You should smell the garlic first, then a light nutty aroma from the flour as it toasts without scorching.

Pour in 1 cup beef broth while scraping up the browned bits from the bottom, then add 3/4 cup whipping cream and let it simmer for 1 to 2 minutes. The sauce should lose its thin look and start coating the back of a spoon.

Put 1/4 cup sour cream in a small bowl and whisk in a few spoonfuls of the hot sauce until smooth, then stir that tempered mixture back into the pan. This should blend in smoothly and keep the sauce creamy rather than grainy or split.

Stir in 1 Tbsp Worcestershire sauce, 1/2 tsp dijon mustard, 1/2 tsp salt, and 1/4 tsp black pepper, then return the beef and any juices from the plate to the pan and warm everything just to a gentle simmer. The finished gravy should look glossy and creamy, and the beef should be heated through without cooking any further.