1. Set the oven to preheat at 350F. Let the butter melt in a pan over medium heat and sauté the onions and garlic for three minutes. Next, toss in the mushrooms and cook for another five to seven minutes, stirring constantly.
2. Arrange two layers of aluminum foil in four places, and place a tuna steak at the center of each foil sheet. Fold the edges so the foil forms a makeshift dish.
3. Scoop a quarter of the sautéed mix onto each tuna steak, sprinkle pepper, salt, and parsley, and add a quarter cup of white wine over each steak.
4. Tent the foil over the tuna steaks and seal the edges tightly. Then, place the foil-wrapped tuna steaks on a baking sheet and toss them in the oven.
5. Bake the tuna steaks for about 15 to 20 minutes, depending on your preferred doneness.

Another impressive oven-cooked tuna steak in foil can be found here.