Gather 6 to 8 asparagus spears into a bundle with the tips together, and wrap a slice of prosciutto securely around the middle. Repeat to make 6 bundles. Brush a rectangular baking dish (large enough to hold the bundles in one layer) with 1½ tablespoons of olive oil. Place the bundles seam side down in the baking dish, arranged side by side, with the tips facing the same way. In a small saucepan, heat the truffle butter and 1½ tablespoons of olive oil until the butter melts. Drizzle it over the bundles. Sprinkle with 3/4 teaspoon fleur de sel, 1/2 teaspoon pepper, and Gruyère and bake for 12 to 15 minutes, until the butter is sizzling and the cheese melts and starts to brown. Serve hot.