Prepare the grill for direct grilling at 500ºF.
Make the hot sauce by combining all 5 ingredients in a blender (or food processor). Blend until smooth. Reserve.
Squeeze the lime over the mahi mahi filet(s), and then brush with olive oil. Season liberally with chili powder and sprinkle with a little salt.
Grill the fish over direct heat, turning once, until the flesh flakes under gentle pressure from your finger or a pair of tongs. Timing will vary with the thickness of the fillet. A thick piece will require about 6 minutes on the first side and 4 on the second. When grilling fish, the fillet will let you know it is ready to turn when it releases easily from the grill. If it sticks, it probably isn’t time to flip.
Grill one side of each tortilla over direct heat for about 20 seconds.
To assemble the tacos, stack 2 tortillas on top of each other, add some flakes of mahi mahi and drizzle with hot sauce. Add a little cabbage and onion and a slice of avocado. Sprinkle with queso fresco and top off with some cilantro leaves. Serve with lime wedges and encourage your guests to use them!