Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with aluminum foil and lightly grease.
Mash together the mackerel, tuna, and hominy in a large bowl until blended. Add the eggs, olive oil, orange bell pepper, jalapeno pepper, onion, garlic, and prunes. Season with salt, black pepper, and red pepper flakes. Stir until evenly combined, then fold in the bread crumbs. Form into 12 patties, and slightly flatten between your palms, place onto the prepared baking sheet.
Bake in preheated oven until hot and lightly browned, about 10 minutes.
While the surprise burgers are baking, toast the whole wheat bread, and spread with honey mustard. Place a lettuce leaf onto the mustard side of the bread. Place each burger between two slices of bread to serve. Serve either warm or cold.