Step 1Cook the onion slices, 1 clove of garlic and the chile on a well-seasoned grill pan over high heat or an outdoor grill until charred on all sides and tender, about 5 minutes. Chop and set aside.
Step 2Drain the beans reserving the liquid. Cook the beans over low heat in a nonstick skillet sprayed with nonstick cooking spray, mashing until pureed. Stir in the onion, garlic, chile and 1/4 teaspoon of salt. Stir in about 1/3 cup of reserved liquid from the beans and simmer several minutes, stirring occasionally. Stir in 2 tablespoons of lime juice.
Step 3Cut the remaining clove of garlic in half. Rub the surface of the halibut with the cut clove. Spray the fish with nonstick cooking spray and rub the oregano over the surface. Cook the fish on a well-seasoned grill pan over high heat or an outdoor grill about 5 minutes. Lightly season with salt and turn. Continue grilling until the fish flakes easily when tested with a fork, about 5 minutes. Season with salt and remove from the grill. Set aside.
Step 4Heat the oven to 425 degrees.
Step 5For the dressing combine the remaining 1 tablespoon of lime juice, the lemon juice, water, chopped cilantro, 1/8 teaspoon salt and pepper to taste.
Step 6Lightly spray both sides of the tortillas with nonstick cooking spray and place in a single layer on baking sheets. Bake until crisp, 8 to 10 minutes. Remove from the oven.
Step 7Spread about 2 tablespoons of the beans on each tortilla. Top each with 1 cup of romaine.
Step 8Flake the fish and divide among the tortillas, placing it on top of the greens. Drizzle the tostadas with the dressing. Sprinkle tomatoes over the top of each.