In a large skillet, heat 1/2 tablespoon extra-virgin olive oil over medium-high. Add 1/2 medium yellow onion. Cut into 1-inch wedges, sute until golden, about 6 minutes. Add 1 smashed garlic clove, 1/2 teaspoon ground coriander, and 1/8 teaspoon red-pepper flakes; cook until fragrant, about 30 seconds. Add 1 small bunch broccoli, cut into florets, stalks peeled and thinly sliced; 1 tablespoon thin lemon-zest strips; and 1 cup water. Bring to a boil, then reduce heat and cover. Cook until fork-tender, about 8 minutes. Season with coarse salt and red pepper flakes, and serve with lemon wedges.