Bring a large pot of water to a boil and salt it generously. Trim the bottom end of the Brussels sprouts, leaving the core intact, and pull off the outer dark leaves. Halve through the core. Add the Brussels sprouts and cook, uncovered, until tender, about 6 minutes. Drain and rinse under cold running water. (This can be done a day ahead.)
Meanwhile, put the oil and bacon in a very large skillet or stewpot, and cook over medium heat, stirring occasionally, until the bacon is crispy. Remove the bacon with a slotted spoon and set aside. Increase the heat to medium-high, add the Brussels sprouts and cook, stirring occasionally, until they brown and the edges get crisp, about 10 minutes. Stir in the vinegar, salt, pepper, and bacon. Serve warm.
3 pounds medium Brussels sprouts
2 tablespoons vegetable oil
8 thick slices bacon (about 8 ounces), cut crosswise into 1/2-inch strips
2 teaspoons apple cider vinegar
1/2 teaspoon kosher salt, plus more as needed
Freshly ground black pepper