Melt 1 tablespoon butter in a skillet over medium heat; cook and stir garlic in hot butter until fragrant, 1 to 2 minutes. Stir broccoli and brussels sprouts into garlic; add tomato and remaining butter. Season with salt and red pepper flakes.
Stir Brussels sprouts mixture until well-combined, cover the skillet, and cook until browned on one side, about 5 minutes. Flip sprouts and broccoli cover the skillet again, and cook until browned on the other side, about 4 minutes.
