Trim the stem ends of the Brussels sprouts and remove any yellow or loose outer leaves. Cut larger sprouts in half lengthwise so halves are similar in size; keep small ones whole.

Cook bacon

Slice bacon crosswise into 1/2-inch pieces. In a large skillet (preferably cast iron) over medium heat, add the bacon pieces and cook, stirring occasionally, until crisp and browned, about 6–8 minutes. Use a slotted spoon to transfer bacon to a paper-towel-lined plate, reserving about 1 to 2 tablespoons of bacon fat in the skillet. If there is more fat, spoon out excess leaving roughly 1–2 tbs.

Sear sprouts

Increase heat to medium-high and add 1 tablespoon olive oil and 1 tablespoon butter to the skillet with the reserved bacon fat. When butter foam subsides and skillet is hot, place Brussels sprouts cut-side down in a single layer. Work in batches if needed to avoid overcrowding. Cook without moving until the cut sides are deeply browned and caramelized, 4–6 minutes.

Finish cooking sprouts

Toss the sprouts, add cooked bacon back to the pan, season with 1 tsp kosher salt and 1/2 tsp freshly ground black pepper, then reduce heat to medium-low. Cover and cook for 3–4 minutes more until sprouts are tender when pierced with a fork but still have some bite. Taste and adjust seasoning.

Balsamic reduction & finishing

Prepare reduction

While the sprouts are searing (or in a small saucepan), combine 1/2 cup balsamic vinegar and 1 tablespoon brown sugar. Mince the garlic clove and add it to the pan with 1/4 tsp red pepper flakes. Bring mixture to a gentle simmer over medium heat, then reduce to medium-low and simmer until syrupy and reduced by about half, roughly 6–10 minutes. Stir occasionally and watch carefully to prevent burning. If it becomes too thick, remove from heat; it will thicken further as it cools.

Finish and glaze

When the Brussels sprouts and bacon are cooked and plated or still in the pan, drizzle 1 teaspoon fresh lemon juice into the reduction (optional) to brighten it. Spoon or drizzle desired amount of balsamic reduction over the sprouts and bacon. Toss gently to coat or leave a decorative drizzle on top.

Garnish and serve

Sprinkle chopped fresh parsley over the finished sprouts (optional) and serve immediately while warm.