Heat the olive oil in a large frypan over medium-high heat. Season the mahi mahi with salt and pepper. Cook the fillets in the hot oil until the fish is golden brown on each side and no longer translucent in the centre, about 3 minutes per side.
Meanwhile, whisk together the sour cream, lime juice, ginger, cumin, cayenne pepper, salt and pepper to taste; set aside. Gently combine the mango, pineapple, avocado and jalapeno in a bowl.
To assemble, place a cooked mahi mahi fillet into the centre of a warmed tortilla. Place a scoop of the mango salsa onto the fish, then drizzle with the sour cream sauce and finish with a generous pinch of chopped coriander.