01 - Warm the olive oil in a large skillet or sauté pan over medium heat.
02 - Add fennel wedges and onion if using, arranging in a single layer. Cook 4 to 5 minutes, turning once, until golden on both sides.
03 - Sprinkle sea salt, black pepper, and red pepper flakes over the fennel and onion.
04 - Pour in vegetable broth, reduce heat to low, cover, and simmer for 25 to 30 minutes until fennel is tender and most liquid has evaporated.
05 - Remove lid, raise heat to medium-high, and cook an additional 2 to 3 minutes to reduce remaining liquid.
06 - Sprinkle grated Parmesan evenly over the fennel, cover, and let stand for 2 minutes to melt the cheese.
07 - Top with chopped parsley and lemon zest before serving.