Mild tilapia and shrimp get a real kick from a marinade of lime juice and fresh herbs. A crisp, spicy cabbage slaw makes the perfect topper. Because tilapia fillets are thicker on one side than the other, they're hard to cook evenly. Here, we split them through the center seam so they can go on the grill separately.
For the marinade: Pulse the lime juice, olive oil, marjoram, cilantro, red pepper flakes, garlic clove and 1/2 teaspoon salt in a mini-food processor or blender until smooth. Cut each tilapia fillet in half through the seam, separating the thicker half from the thinner. Place the tilapia and shrimp in a large resealable plastic bag and add the marinade. Toss to coat, seal and refrigerate for 1 hour.
For the slaw: Meanwhile, toss the cabbage with 1/2 teaspoon of salt in a large bowl and let stand at room temperature for 30 minutes to soften it a bit. Whisk the sour cream, jalapenos and pickling liquid in another large bowl. Toss in the seasoned shredded cabbage and stir to coat. Season with black pepper, cover and set aside.
Prepare a grill for medium-high heat. Transfer the tilapia and shrimp to a rimmed baking sheet and sprinkle with 1/2 teaspoon salt and a few grinds of pepper. Brush the fillets with 1 tablespoon vegetable oil. Lightly oil the grill grates.
For the taco bar: Wrap the tortillas in foil and set to the side on the heated grill while you grill the tilapia and shrimp.
Grill the tilapia starting with the thicker pieces, 4 minutes for the thicker fillets, 3 minutes for the thinner ones. Use a spatula to loosen the fish and gently flip. Grill until just cooked through, another 2 to 3 minutes. Grill the shrimp until opaque, 2 to 3 minutes, flipping halfway. Serve with the tortillas, slaw, scallions, radishes and salsa.
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