Preheat your oven to 425°F (220°C). Rinse the Brussels sprouts, trim the ends, and quarter each sprout, reserving any loose leaves.

Toss the quartered Brussels sprouts with olive oil, salt, and pepper in a bowl. Spread in a single layer on a baking sheet. Roast for 25-30 minutes, stirring halfway through.

Slice the bacon into ½-inch pieces and cook in a skillet over medium heat until crispy, about 8-10 minutes. Drain on paper towels, leaving some grease in the skillet.

In a saucepan, simmer balsamic vinegar for 10-12 minutes until it thickens and reduces to about 3 tablespoons.

Sauté diced yellow onion in the skillet with bacon grease for 2-3 minutes until translucent. Add crispy bacon to integrate.

Combine the roasted Brussels sprouts with the onion and bacon mixture. Drizzle with the balsamic reduction and toss gently to serve warm.