Place refried beans in small pan or pot with about 1/3 cup water and some hot sauce. Reheat beans and reduce heat to low to keep warm.
Combine cabbage with onions and dress with juice of 2 limes and some salt, then reserve or chill. Cut remaining lime into small wedges to pass at table for topping tacos.
Preheat oven to 250F and arrange a cooling rack over a baking sheet to keep fish crisp in oven while frying up batches.
Pour about 2 inches of fry oil into a large heavy pot such as a Dutch oven. Heat over medium to medium-high heat. When oil is ready for frying bubbles will surround and flow from a submerged wooden spoon handle.
Meanwhile, pat fish dry and season with Old Bay and salt.
Whisk together flour and cornstarch, sesame seeds, chili powder, a couple of healthy pinches of salt, beer and egg.
Pour a little more flour onto a plate for dredging. Dust fish in plain flour then dip in batter and fry to golden brown and crispy, about 3-4 minutes. Do not crowd the pot and allow oil to reheat before frying second batch of fish. Place cooked fish in oven to keep crisp.
Heat a small, dry skillet over medium-high heat and heat the corn tortillas.
To eat, use a healthy dab of spicy refried beans to glue 2 tortillas together. This will prevent ripping of the soft corn tortillas. Fill tacos with cabbage, fish, salsa or chopped tomatoes, sour cream or green-garlic sour cream and garnish with hot peppers.