- With the rack in the middle position, preheat the oven to 190°C (375°F).
- Line a baking sheet with aluminum foil.
- Roll out the dough into 4 10-cm (4-inch) squares. Place the squares on the baking sheet. Bake until golden, about 15 minutes. Cool to room temperature, then cut the squares in half horizontally. Set aside.
- Meanwhile, in a skillet over medium heat, soften the onion and garlic in the oil. Add the mushrooms and cook until they release their water and begin to brown, 12 to 15 minutes. Season with salt and pepper. Deglaze with the vin-egar and water. Adjust the seasoning. Keep warm.
- In a skillet over high heat, fry the steaks in the oil and butter to the desired degree of doneness. Season with salt and pepper.
- Spread the liver pâté on the bottom halves of the pastry squares. Divide the duxelles among the pastry squares and place the steaks on top. Cover with the top halves of the pastry squares.