Warm a large, heavy-bottomed skillet or pot over medium-high heat. Add 1 tablespoon of butter and allow it to melt until slightly foamy. Season the steak cubes with salt and pepper, then add them to the pan in a single layer. Sear the meat for about 8 minutes, turning occasionally until each piece is evenly browned. Once done, transfer the steak to a bowl, discard any excess liquid from the pan, and set the meat aside.

In the same pan, melt another tablespoon of butter over medium heat. Add the sliced onions and cook for about 2 minutes, stirring occasionally until they begin to soften and release their natural sweetness. Add the mushrooms, then reduce the heat to medium. Continue cooking for about 30 minutes, stirring every 5 to 10 minutes, until the onions and mushrooms are deeply caramelized and golden brown.

Stir in the minced garlic, dried thyme, and Worcestershire sauce. Cook for about 1 minute, just until the garlic becomes aromatic and blends with the rich flavors in the pan. Return the browned steak pieces to the skillet, then add the remaining butter. Stir everything together, allowing the butter to melt and coat the meat and vegetables evenly. Let the mixture cook for an additional 5 minutes, giving the flavors time to marry beautifully.

Serve hot with creamy mashed potatoes, steamed rice, or a light cauliflower rice for a low-carb option. Garnish with freshly chopped parsley for a bright finishing touch, and enjoy the deep, comforting flavors of this French-inspired steak dish.