Preheat your oven to 400°F (200°C).
Cut four large pieces of aluminum foil, about 12×12 inches each.
In a large bowl, combine the steak cubes, sliced mushrooms, and red onion.
Drizzle the olive oil over the mixture, then sprinkle with garlic powder, dried thyme, salt, and pepper. Toss everything together until well coated.
Divide the steak and mushroom mixture evenly among the four pieces of foil.
Place a tablespoon of butter on top of each portion.
Fold the foil over the mixture and seal the edges tightly to create a packet.
Place the foil packets on a baking sheet and bake in the preheated oven for 20-25 minutes, or until the steak is cooked to your desired doneness.
Carefully open the packets (watch out for steam) and garnish with fresh parsley if desired before serving.
Variations & Tips
For a bit of heat, add a pinch of red pepper flakes to the seasoning mix. If you have picky eaters, you can customize each packet with different vegetables, such as zucchini or bell peppers. For a more indulgent version, sprinkle some shredded cheese on top of the steak and mushrooms before sealing the packets. If you prefer a different protein, chicken breast or pork tenderloin can be used in place of the steak.
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