Halve, core and slice 3 fennel bulbs 1/2 inch thick. Boil in salted water with 1/2 lemon until tender, about 15 minutes; drain. Arrange in a buttered 2-quart baking dish. Sprinkle with a mix of 1/4 cup each parmesan and panko, 3/4 teaspoon chopped fennel seeds, 1/2 teaspoon kosher salt and a few grinds of pepper; dot with 3 tablespoons butter. Broil until golden, about 3 minutes. Top with chopped fennel fronds.