Brussels Sprouts with Pancetta and Onions

1 1/2 lb. Brussels sprouts, trimmed and halved

4 oz. pancetta, diced

1 large onion, quartered and thinly sliced crosswise

1/2 tsp. kosher salt

1/4 tsp. freshly ground black pepper

2 T. red wine vinegar (optional)

 

Bring 1-inch of lightly salted water to a boil in a large, deep skillet. Add sprouts and simmer, covered, until crisp-tender, 6 to 8 minutes. Drain and run under cold water to cool. Wipe out the skillet. Add pancetta and cook over medium heat, stirring occasionally, until crisp, 5 to 7 minutes. With a slotted spoon, transfer to paper towel–lined plate. Add onion to drippings in skillet and cook, stirring occasionally, until tender and golden, 8 to 10 minutes. Increase heat to medium-high. Add sprouts, salt and pepper, and cook, tossing, until heated through, about 3 minutes. Add pancetta and vinegar, if using, and toss to combine. Use leftovers in an omelet the next morning.