1. Cook the asparagus in a large sauce-pan of boiling salted water until tender, about 8 minutes. Using tongs, transfer the asparagus to a colander and leave the pan of water simmering on the stove.
2. Melt 1 tablespoon of the butter in a large skillet. Add the prosciutto and cook over low heat until crisp, about 4 minutes. Transfer the prosciutto to a plate.
3. Melt 2 tablespoons of the butter in the skillet. Add the scallops, season with salt and pepper and cook over moderately high heat until browned, about 2 minutes per side. Transfer the scallops to a plate.
4. Add the lemon zest to the skillet and cook over moderate heat until browned, about 1 minute. Add the lemon juice and simmer for 10 seconds. Add the stock and simmer, scraping the bottom of the skillet, until reduced to a rich glaze, about 3 minutes. Swirl in the remaining 1/2 tablespoon of butter. Return the scallops, along with any juices, to the skillet and cook over low heat until heated through.
5. Return the asparagus to the simmering water to heat through. Season the sauce with salt and pepper. Using tongs, transfer the asparagus to a large platter and spoon the scallops and sauce over it. Top with the fried prosciutto and serve.