Place broccolini in a large skillet with about 2 inches of water; bring to a boil and cook until bright green, 1 to 2 minutes. Drain.
Heat olive oil in the same skillet over medium heat. Stir in garlic and cook until golden and fragrant, 1 to 2 minutes. Add broccolini; cook and stir until heated through, 2 to 3 minutes.
Squeeze lemon juice over broccoli and season with red pepper flakes, salt, and pepper.
