Position a rack in the center of the oven and heat the oven to 250 degrees F. Put the bread in a food processor and pulse until coarse crumbs form. Spread the crumbs on a rimmed baking sheet; bake until they're dry and lightly crisped, about 20 minutes. Let cool and then grind again in the processor to get a scant 1/4 cup of fine crumbs.

Melt the butter in a skillet over moderately low heat. Add the breadcrumbs and thyme. Cook slowly, stirring often, until the crumbs are a uniform deep golden brown, about 18 minutes. Season with 1/8 teaspoon salt and set aside to let cool. The crumbs will crisp as they cool.

Raise the oven temperature to 450 degrees F. Put the asparagus on a baking sheet in a single layer and drizzle with the olive oil. Using your hands, toss the spears to coat evenly with the oil. Season with 1/2 teaspoon salt. Roast until the spears are tender and lightly blistered, 12 to 14 minutes for medium spears. Transfer them to a platter or individual plates, sprinkle with the lemon juice, and top with the breadcrumbs. Serve immediately.