Asparagus with Morels and Tarragon

Cook white and green asparagus separately in a 5- to 6-quart pot of boiling salted water , uncovered, until just tender, about 5 minutes for white asparagus; about 3 minutes for green asparagus.

Transfer asparagus with a slotted spoon to a colander and rinse under cold water (to stop cooking), then drain again. Pat dry.

Heat 3 tablespoons butter in a 12-inch heavy skillet over moderately high heat until foam subsides, then sauté shallots, stirring, until golden brown, about 2 minutes. Add morels and sauté, stirring frequently, until tender, about 5 minutes. Add asparagus, tarragon, salt, pepper, and remaining 3 tablespoons butter and sauté, stirring, until heated through, about 3 minutes. Remove from heat and stir in lemon juice. Serve immediately.