Remove pastry shell from oven, and sprinkle with Gruyere. Trim the bottoms of the asparagus spears to fit crosswise inside the tart shell; toss in olive oil then arrange in a single layer over Gruyere. Season with salt, pepper, and garlic powder. Bake until spears are tender, 20 to 25 minutes. After, lightly drizzle with good quality truffle oil.