Place beef into a large bowl. Stir in red wine, salt, and black pepper. Marinate for 10 minutes, then remove beef and pat dry with a paper towel. Reserve remaining marinade.

Heat olive oil in a large skillet over medium heat. Stir in beef; cook and stir until browned, 5 to 7 minutes. Transfer beef to a plate, then drain any remaining grease from the skillet.

Melt 2 tablespoons butter in the skillet over medium heat. Stir in onion, garlic, and a pinch of salt. Cook and stir until onion is soft and translucent, 3 to 4 minutes. Transfer onion mixture to the plate with beef; set aside.

Melt 2 tablespoons butter in the skillet over medium heat. Add mushrooms; cook and stir until tender, about 10 minutes. Transfer mushrooms to a bowl; set aside.

Melt 1/4 cup butter in the skillet over medium heat. Whisk in flour; cook and stir until flour no longer tastes raw, about 4 minutes. Slowly whisk in beef stock. Bring to a boil, stirring constantly, then reduce heat to medium-low.

Pour in reserved marinade, Worcestershire sauce, red pepper flakes, and mustard, then add beef and onion mixture. Cover and simmer until meat is tender, about 1 hour. Season with salt and black pepper.