Preheat the oven to 350 degrees F (180 degrees C).
Peel and prepare the breadfruit by cutting off the stem, slicing into wedges, peeling the skin, and removing the core.
Boil wedges in salted water for 20 minutes, or until tender. Drain and cool slightly.
In a saucepan, melt butter over low heat. Whisk in flour and cook for 4–5 minutes to form a smooth roux.
Slowly whisk in evaporated milk. Add parsley, onion, mustard, scotch bonnet (if using), nutmeg, black pepper, and cheeses (reserve 1/4 cup cheddar for topping). Stir until thick and smooth.
Slice boiled breadfruit into 1/4-inch thick slices. Layer half in a greased baking dish, then pour over half the cheese sauce. Repeat with remaining breadfruit and sauce.
Sprinkle top with reserved cheddar (and breadcrumbs if using).
Slice each wedge of cooked breadfruit about 1/4 inch thick, then place a layer onto a greased cooking dish. Add a layer of cheese sauce, then another layer of sliced breadfruit and top with the remaining cheese sauce.
Rest 5 minutes before serving.