10 oz. slab bacon, cut into matchstick-size pieces
2 Tbs. unsalted butter, divided
3 medium yellow onions, sliced thin
2 lbs. Brussels sprouts, bottoms trimmed, outer leaves removed, and larger sprouts cut in half
3 Tbs. good quality sherry vinegar
Kosher salt, to taste
Freshly cracked black pepper, to taste
Put a large pot of salted water on to boil over medium-high heat.
Cook bacon in a large cast-iron skillet over medium heat, stirring frequently, until the fat has been rendered and bacon is crisp, about 25 minutes. Remove bacon to paper towels. Reserve two tablespoons of fat and discard the rest. Return the two tablespoons of fat back to the skillet. Add one tablespoon butter and stir over medium heat until melted and the combined fats are hot. Add onions and cook, stirring occasionally, about 25 minutes, or until tender and golden brown. Set aside.
Add Brussels sprouts to the pot of boiling water and cook for five minutes, or until they are tender when pierced with a knife. While the Brussels sprouts are cooking, make an ice bath by putting equal parts ice and water in a large bowl. When the sprouts are done, use a spider skimmer or slotted spoon to remove them from the pot and put them immediately into the ice bath to stop further cooking and retain the green color. Once chilled, remove them to paper towels to drain.
When ready to serve the dish, place skillet over medium-high heat and add bacon and onion mixture. Add vinegar, using it to loosen any bits on the bottom of the skillet. Cook, stirring, for a couple of minutes, until vinegar evaporates.
Stir in Brussels sprouts and heat. Add remaining tablespoon of butter and stir to incorporate. Season to taste with salt and pepper. Serve.