Cook bacon in large skillet until crisp; transfer to paper towels. Pour off all but 1 teaspoon drippings from skillet. Return bacon to skillet; mix in onion. Remove from heat. Whisk oil and lemon juice in small bowl; season with salt and pepper.
Cook asparagus in pot of boiling salted water until crisp-tender, about 3 minutes. Transfer to paper towels to drain, then place in large bowl. Add dressing; toss. Arrange on platter. Stir bacon mixture over medium-low heat about 3 minutes; spoon atop asparagus.
