Step 1

Stir garlic, orange juice, oregano, 1 cup onion, ¼ cup cilantro, ¼ cup oil, and 3 Tbsp. lime juice in a medium bowl. Season fish with salt and pepper. Spread half of onion mixture over bottom of 11x7x2" glass baking dish. Arrange fish over onion mixture. Spoon remaining onion mixture over fish. Cover and chill 30 minutes. Turn fish; cover and chill 30 minutes longer.

Step 2

Whisk mayonnaise, milk, and remaining 2 Tbsp. lime juice in a small bowl.

Step 3

Prepare a grill for medium-high heat; brush grate with oil. Grill fish, with some marinade still clinging, until just opaque in the center, 3–5 minutes per side. Grill tortillas until slightly charred, about 10 seconds per side.

Step 4

Coarsely chop fish; arrange on a platter. Serve with lime mayonnaise, tortillas, avocados, cabbage, Salsa Verde, lime wedges, and remaining 1 cup chopped onion and ½ cup cilantro.