1. Cook bacon in a 12-inch heavy skillet over moderately high heat, stirring occasionally, until browned and crisp, about 6 minutes. Transfer bacon with a slotted spoon to paper towels to drain. Pour off all but 2 tablespoons fat from skillet, then add oil and onion to skillet and cook, stirring occasionally, until onion is golden, about 4 minutes.
2. Add asparagus and saute, stirring, until crisp-tender, 4 to 5 minutes. Season with salt and pepper.
3. While onion is cooking, cook pasta in a 6- to 8-quart pot of until al dente. Reserve 1 cup pasta cooking water and drain pasta in a colander.
4. Return pasta to pot along with asparagus mixture, 1/2 cup pasta cooking water, and cheese. Add remaining 1/2 cup cooking water to skillet and deglaze by boiling, stirring and scraping up brown bits, 1 minute. Add to pasta and toss over moderately low heat until combined well, about 30 seconds.
5. Serve pasta sprinkled with bacon and with additional cheese on the side.

Yield: 4 servings