Make the fennel slaw by combining cabbage, onions, cilantro, fennel, mayo and lemon juice in a medium bowl.
Heat a char-grill pan or a grill over high heat. Brush the fish with olive oil and season with salt, pepper and chili flakes. Grill for 2-3 minutes per side or until cooked through. Place the tortillas on the grill and turning once until they puff and char a bit, about 1 minute.
Place the fish on the tortillas and top with the slaw. Serve with lime wedges.
A mandoline makes slicing the cabbage and fennel easy! It also keeps it really nice and thin.