2 tablespoons coarse sea salt
4 (10- to 12-ounce) whole trout, cleaned and skin scored
¼ cup yellow mustard
2 tablespoons Worcestershire sauce
1 tablespoon garlic powder
½ medium-size red onion, thinly sliced (about 1 cup)
1 cup vegetable oil
Lime wedges, rice, and salad, for serving
Rub 1 1/2 teaspoons salt evenly over outside and inside of each fish. Stir together mustard, Worcestershire sauce, and garlic powder in a small bowl. Spread evenly over outside and inside of each fish, covering completely.
Stuff cavities of fish evenly with onion slices. Heat oil in a large, deep skillet over medium-high to 350°F. Carefully place 2 fish in hot oil, and cook until skin is browned and crispy, about 4 minutes. Flip fish, and cook just until flesh is white throughout, 2 to 3 minutes. Transfer to a wire rack. Repeat with remaining 2 fish.
Serve fish with lime wedges, rice, and salad.