In a food processor, combine the first eight marinade ingredients until the cilantro and garlic are finely chopped. Pour the marinade into a resealable plastic bag. Season the mahi-mahi with salt and pepper and add to the bag. Seal and chill for 20 minutes. Rinse out the food processor bowl.
Heat a griddle or grill pan over medium-high.
Place the cabbage in a small serving bowl.
In the food processor, combine the avocados, crema and juice of 1 lime. Season with salt; puree. Transfer the avocado sauce to a bowl.
In a medium bowl, combine the tomatoes, chopped red onion, chopped jalapeno and chopped cilantro to make pico de gallo; season with salt.
In a medium bowl, combine the sliced onion, sliced jalapeno, the juice of 1 lime and the agave. Season liberally with salt and pepper and toss to combine. Let stand a few minutes until juices form. Toss the pickled onions again before serving.
Working over a gas burner or in the preheated griddle pan, cook the tortillas until charred, about 30 seconds on each side. Wrap the tortillas in a cloth napkin or towel to keep warm.
Add the fish and the halved lime, cut sides down, to the griddle pan. Grill the lime until charred, 3 to 4 minutes. Grill the fish until opaque in the center and firm and browned at the edges, turning once, 7 to 8 minutes. On a serving platter, coarsely break up the fish; douse with the juice of the grilled lime.
Build the tacos with the tortillas, fish, cabbage, avocado sauce, pico de gallo, pickled onions and sliced radishes.