1 cup balsamic vinegar
1 cup dry red wine
1/2 cup sugar
1 1/2 pounds sliced button mushrooms
3 tablespoons butter, divided
1/4 cup duck or goose liver pâté
1 tablespoon chopped fresh thyme plus 6 sprigs for garnish
6 frozen puff pastry shells (one 10-ounce package)
1 tablespoon canola oil
6 4-ounce filet mignon steaks
Coarse kosher salt
2 5-ounce bags fresh baby spinach
This step can be completed up to five days in advance: Bring balsamic vinegar, red wine, and sugar to boil in heavy medium saucepan over high heat, stirring to dissolve sugar. Reduce heat to medium-high and boil until reduced to 3/4 cup, about 14 minutes.
This step should be completed two hours in advance: Let pastry shells stand at room temperature. Preheat oven to 400°F. Place pastry shells on baking sheet, spacing 1 inch apart. Bake until puffed and deep golden brown, about 23 minutes. Cool.
Separate mushrooms into two batches and finely chop in processor. Melt 2 tablespoons butter in heavy large skillet over medium-high heat. Add mushrooms and sprinkle with salt. Cook until almost all liquid evaporates, stirring often, about 10 minutes. Add pâté and chopped thyme, stir to blend. Season to taste with salt and pepper
Heat oil in heavy large skillet over medium-high heat. Sprinkle steaks generously with coarse salt and pepper. Cook steaks until browned and cooked to desired doneness, about 4 minutes per side for medium-rare. Transfer to plate and let the dish rest for 10 minutes.
Melt remaining 1 tablespoon butter in heavy large pot over medium heat. Add spinach, sprinkle with salt and pepper, and toss just until spinach wilts, about 3 minutes.
Rewarm sauce and mushroom mixture. Halve puff pastry shells horizontally. Place 1 pastry shell bottom on each of 6 plates. Top with rounded tablespoon mushroom mixture. Divide spinach among shells, then top with steak. Place thyme sprig atop each steak. Drizzle sauce around and over steak and prop pastry shell top alongside.