Preheat the oven to 450 degrees F (230 degrees C).

Fill a large pot with water and bring to a rolling boil. Cook Brussels sprouts in boiling water for 2 minutes. Use a slotted spoon to remove sprouts to a bowl of ice water to cool. Drain, pat dry, and set aside.

Place onions in boiling water; cook until nearly tender, about 10 minutes. Drain, then rinse with cold water. Pat dry and set aside.

Melt butter in an oven-proof skillet over medium heat. Stir in Brussels sprouts and onions; toss with butter to coat. Stir in salt and black pepper; cook for 2 minutes, stirring occasionally.

Transfer the skillet to the preheated oven and roast until vegetables are golden and tender, 15 to 20 minutes. Garnish with lemon wedges to serve.