Prep the Brussels Sprouts: Start by trimming the ends and halving the Brussels sprouts for uniform cooking.
Cook the Onion: In a skillet over medium heat, add the reserved bacon grease (or olive oil). Sauté the sliced onion until it's soft and translucent, about 5 minutes.
Add Brussels Sprouts: Toss in the halved Brussels sprouts. Stir well to coat them in the delicious onion-infused grease.
Sauté to Perfection: Cook for about 10-15 minutes until the sprouts are tender and crispy, stirring occasionally to ensure even cooking.
Finish with Parmesan: Sprinkle shredded Parmesan cheese over the top, letting it melt slightly before serving.