Preparation Cook asparagus in large pot of boiling salted water until just crisp-tender, about 4 minutes. Drain; rinse under cold water. Drain; pat asparagus dry. (Can be prepared 1 day ahead. Wrap in paper towels and plastic bags; refrigerate.) Toss mushrooms with lemon juice in medium bowl; season with salt and pepper. Whisk cream and mustard in small bowl to blend; gradually whisk in oil. Let mushroom mixture and dressing stand 30 minutes or cover and refrigerate up to 2 hours. Arrange asparagus on platter. Whisk tarragon into dressing. Drain mushrooms well and return to bowl. Mix in dressing. Season to taste with salt and pepper. Spoon mushroom dressing over asparagus and serve.
Preparation Cook asparagus in large pot of boiling salted water until just crisp-tender, about 4 minutes. Drain; rinse under cold water. Drain; pat asparagus dry. (Can be prepared 1 day ahead. Wrap in paper towels and plastic bags; refrigerate.) Toss mushrooms with lemon juice in medium bowl; season with salt and pepper. Whisk cream and mustard in small bowl to blend; gradually whisk in oil. Let mushroom mixture and dressing stand 30 minutes or cover and refrigerate up to 2 hours. Arrange asparagus on platter. Whisk tarragon into dressing. Drain mushrooms well and return to bowl. Mix in dressing. Season to taste with salt and pepper. Spoon mushroom dressing over asparagus and serve.