Cook pasta according to package directions; drain. Cover and keep warm.
Cook pancetta over medium heat in a large skillet until crisp. Using a slotted spoon, transfer pancetta to paper towels to drain, reserving 1 tablespoon drippings in skillet. Add onion and garlic to drippings in skillet; cook and stir for 4 to 5 minutes or until tender.
Whisk together milk and flour in a small bowl. Add milk mixture to skillet. Cook and stir until thickened and bubbly. Cook and stir for 2 more minutes. Add canned peas and pancetta to the cooked pasta; gently toss to coat.
To serve, sprinkle pasta with cheese and pepper. If desired, top with sage.