Adjust oven rack to about 4 inches from heating element and heat broiler.
Toss asparagus with oil and salt and pepper, then lay spears in single layer on heavy rimmed baking sheet. Broil, shaking pan halfway through to turn spears, until asparagus is tender and lightly browned, 8 to 10 minutes.
Cool asparagus 5 minutes and arrange on serving dish.
Fry bacon strips in medium skillet over medium heat until brown and crisp, about 6 minutes. Using slotted spoon, transfer bacon to paper towel–lined plate and set aside.
Whisk onion, vinegar, parsley, and oil in small bowl; season to taste with salt and pepper. Drizzle over asparagus, sprinkle with bacon, and serve immediately.