Preheat the oven to 400 degrees.
Fill a large pot with water, add 1 tablespoon salt, and bring to a boil.
If the asparagus are thick, peel them halfway up the stalk with a vegetable peeler.
Immerse the asparagus in the boiling water and cook for 2 minutes.
Drain the asparagus and put them immediately into a large bowl of ice water to set the bright green color.
Allow to chill for 2 to 3 minutes.
Drain and pat the asparagus dry.
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 ¾ teaspoon fleur de sel, ½ teaspoon pepper, and the Gruyère and bake for 12 to 15 minutes, until the butter is sizzling and the cheese melts and starts to brown.
Serve hot.
