For the batter, whisk together 2 cups all-purpose flour, 1 1/2 teaspoon baking powder, 1/2 teaspoon granulated garlic, 1/4 teaspoon cayenne, 1/2 teaspoon dry mustard, 1/2 teaspoon dried whole Mexican oregano, rubbed to a powder, Kosher salt to taste, and 1/2 teaspoon freshly ground black pepper in a large bowl until well blended. Stir in 1 bottle (12 oz.) cold beer until there are no lumps. Cut 2 pounds firm, meaty fish such as halibut or snapper into finger-shaped pieces and sprinkle them with some lime juice and salt. Pat dry. Dip the pieces into the batter, let the excess drip, and deep fry the battered fish pieces until golden; drain on paper towel-lined rack.
Preheat oven to 400 degrees. Place two egg whites in a shallow bowl and lightly beat with a fork. In another shallow bowl, place 1/2 cup coarse cornmeal or grits seasoned with salt and pepper. Pat dry four 4-oz. servings of white fish, such as tilapia or halibut, and dip each piece of fish first in the egg whites and then dredge in the cornmeal. Place on a parchment paper-lined baking sheet or baking dish coated with cooking spray. Bake at 400 degrees for about 20 minutes, turning once mid-way through.
Place 24 large or jumbo shrimp, peeled and deveined, in a large bowl with 1 tbsp olive oil and salt and pepper to taste. Toss well to coat. Heat 1-2 tablespoons oil in a large skillet over medium-high heat. Add shrimp and cook 5-6 minutes, turning to ensure that they brown evenly on both sides.
Char a fresh poblano pepper over an open gas flame, turning occasionally, until thoroughly blistered on all sides. Wrap tightly in aluminum foil and let rest for 15 to 20 minutes. Remove and gently scrape off the skin. Halve, stem and seed the pepper. Cut into strips, then dice and place in a bowl. Heat 1 teaspoon oil in a medium skillet over medium-high heat. Add the kernels cut from one large ear of corn and cook 3-4 minutes until slightly charred yet crisp. Add to the bowl with the poblano pepper. Peel and dice 1 small avocado, sprinkle with a little lime juice to prevent it from turning brown, and add to bowl. Add fresh cilantro, season with salt, and toss gently.