Put the shallot and mustard seeds in a small bowl. Combine the vinegar, sugar and thyme sprigs in a small saucepan and bring to a boil. Reduce the heat and simmer, stirring, until the sugar dissolves, 1 to 2 minutes. Pour the vinegar mixture over the shallot mixture and stir. Refrigerate 30 minutes.
Preheat the broiler. Spread the asparagus on a rimmed baking sheet; drizzle with the olive oil and sprinkle with the thyme leaves. Season with 1/2 teaspoon salt and a few grinds of pepper and toss until coated. Broil until crisp-tender and lightly browned in spots, about 4 minutes.
Transfer the asparagus to a platter. Discard the thyme sprigs from the shallot mixture and spoon over the asparagus. Serve with the prosciutto.
Photograph by Con Poulos
Courtesy of Food Network Magazine