How to Make It
Preheat oven to 450°F. Process whitefish in a food processor until mostly smooth. Transfer to a large bowl. Finely dice 1 jalapeño; add to bowl with fish. Stir in mayonnaise, scallions, parsley, 1 teaspoon salt, 1/2 teaspoon black pepper, and cayenne. Gently fold in crabmeat, leaving lumps intact. Divide mixture into 8 portions (about 1/3 cup each); shape into 3-inch-wide, 1/2-inch-thick patties. Sprinkle both sides of patties generously with panko to coat completely; press lightly to adhere. Refrigerate 30 minutes.
While patties chill, stir together tomatoes, shallots, garlic, 1 tablespoon oil, and remaining 2 jalapeños in an ovenproof skillet. Roast in preheated oven until tomatoes collapse, about 15 minutes. Remove skillet from oven, and transfer to stovetop over medium. Add stock, and cook, stirring often, until most of the liquid has evaporated and mixture is reduced to about 1 1/2 cups, about 15 minutes. Transfer mixture to a blender. Secure lid on blender, and remove center piece to allow steam to escape. Place a clean towel over opening. Process, gradually adding butter pieces through opening, until mixture is smooth and butter is combined. Season with 1/2 teaspoon salt and 1/4 teaspoon black pepper; keep sauce warm.
Heat 1/4 cup oil in a large skillet over medium-high. Working in batches, cook crab cakes until golden brown, 2 to 3 minutes per side. Transfer to a wire rack.
Toss herbs with remaining 1 tablespoon olive oil, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper. Spread about 1/3 cup sauce on each of 4 plates; top evenly with herb salad and crab cakes. Serve with lemon wedges.