In a small bowl, combine the lime juice, zest, garlic and olive oil. Season with salt and pepper, to taste. Put the fish in a baking dish and pour the marinade on top. Marinate for no more than 15 to 30 minutes. Meanwhile, preheat the broiler or grill. Lightly coat a baking sheet with olive oil. Remove the fish from the marinade and arrange on the baking sheet, making sure to leave space between the pieces. Broil until just cooked through, about 4 to 5 minutes.
Meanwhile, in a medium saute pan, heat the vegetable oil over medium heat. Fry the corn tortillas, 1 at a time, just until crisp, flipping halfway through to brown both sides. Drain on paper towels and salt while still hot.
To assemble the tostadas: Layer each tostada first with Sauteed White Beans, then the fish. Top with the shredded cabbage and Pineapple and Avocado Salsa. Garnish with a dollop of sour cream and a squeeze of a lime wedge.
Sauteed White Beans:
In a small pan, over medium heat, add the oil. When the oil is hot add the onion with a pinch of salt and saute until soft, about 5 minutes. Add the garlic and cumin and cook until fragrant, about 2 minutes. Raise the heat and add the white wine. Cook until the wine has reduced by half. Stir in the beans and oregano and season with salt and pepper, to taste. Cook gently for 15 minutes. Transfer to a serving bowl and serve.
Pineapple and Avocado Salsa:
In a medium bowl, make the dressing by mixing 2 tablespoons of pineapple juice with the lime juice, honey, oil and jalapeno. Season with salt and pepper, to taste. Add the pineapple and onion. Gently fold in the avocado and cilantro. Let the flavors blend for at least 15 minutes before serving.