Preheat a large cast-iron skillet or griddle over high heat. Place the broccoli in the skillet and char until blistered on one side. Turn and char the other side. Transfer to a large heatproof bowl.
In a large skillet over medium heat, warm the olive oil. Add the garlic and pepper flakes and cook, stirring frequently, until the garlic is golden brown, about 2 minutes. Pour the oil over the broccoli and turn to coat. Let marinate for 10 minutes. Drizzle with the lemon juice, season with the sea salt and lemon zest, and serve hot or at room temperature.