Bring first 3 ingredients 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. DO AHEAD: Sauce can be made 5 days ahead. Cover and chill. Using on/off turns and working in 2 batches, finely chop mushrooms 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. DO AHEAD: Can be made 2 hours ahead. Let stand at room temperature. Preheat oven to 400 degrees. Place pastry shells on baking sheet, spacing 1 inch apart. Bake until puffed and deep golden brown, about 23 minutes. Cool. 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; let rest 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. Prop pastry shell top alongside. Spoon balsamic roasted vegetables and creamed onions with bacon and herbs alongside and serve.