Preheat the oven to 375° F
Remove the tough outer layer of the fennel bulbs. Cut the fennel crosswise into 1/2" slices.
Bring a large pot of water to a boil. Add salt and fennel slices and boil for 2 minutes. Drain the fennel and put it into a bowl of ice water to stop the cooking. Drain the fennel again and pat it completely dry.
Lightly oil a baking dish and layer the fennel to a depth of 1½ inches.
In a small bowl combine the 3 tablespoons of olive oil, fennel seeds, garlic, rosemary, and Aleppo pepper. Drizzle 2 tablespoons of the mixture over the fennel and then sprinkle the whole rosemary leaves over the fennel. Sprinkle the shredded Gruyère over the fennel followed by the Panko bread crumbs. Drizzle the remaining oil mixture over the top, followed by the grated Parmesan.
Bake the fennel uncovered for 25 minutes or until golden brown. Garnish with parsley.