Trim the stem of each Brussels sprout and cut in half if larger than ½-inch diameter. Drizzle trimmed and cut sprouts with oil and sprinkle with salt and pepper. Toss to coat and pour onto a baking sheet in an even layer. Roast in the preheated oven until tender-crisp, 12-15 minutes. A knife inserted will meet a little resistance. Remove from the oven.
2 lb Brussels sprouts, 1 tbsp olive oil, 1 tsp salt, ½ tsp black pepper
While roasting, cook bacon in a large skillet over medium-high heat. Transfer cooked bacon to a towel-lined pan. When all bacon is cooked, leave bacon grease in skillet and remove from heat if the Brussels have not finished roasting.
½ lb thick-cut bacon
Transfer half of the roasted Brussels sprouts to the hot skillet. Fry the sprouts until crispy and browned, 3-5 minutes. Transfer to a towel-lined tray and continue frying the second batch of sprouts. If skillet seems dry, add 2 tablespoons of neutral oil. Finish frying sprouts and transfer to a serving plate.
For the aioli
In a large measuring glass, combine the egg, mustard, lemon zest, lemon juice, salt, pepper, and garlic cloves.