01 - Warm olive oil in a large skillet or sauté pan over medium heat.
02 - Add fennel wedges and onion if using in a single layer; cook for 4 to 5 minutes, turning once until lightly golden on both sides.
03 - Sprinkle sea salt, black pepper, and red pepper flakes over vegetables.
04 - Pour in vegetable broth, reduce heat to low, cover, and simmer for 25 to 30 minutes until fennel is tender and most liquid evaporates.
05 - Remove lid, increase heat to medium-high, and cook for 2 to 3 minutes to concentrate remaining liquid.
06 - Evenly sprinkle Parmesan cheese over fennel, cover, and let sit for 2 minutes until cheese melts.
07 - Top with chopped parsley and lemon zest before serving.