Put 2 tablespoons of oil into a heavy skillet over medium-high heat and cook until the oil is hot, but not smoking. Dredge the mahi mahi filets in blackening spice and cook for 3-4 minutes on each side, until bronzed on the outside. Set aside.
To serve, place a portion of the mahi mahi in the center of each tortilla. Add onion, cilantro, and salsa to taste.