For the mojo:
1) Put the garlic and olive oil in a small microwave-safe bowl, cover with plastic, and microwave on HIGH until the garlic is soft and aromatic, 1 to 2 minutes. Stir in the lime juice, coriander leaves, and salt. Set aside until ready to serve.
For the mahi-mahi:
1) Prepare an outdoor barbecue to medium high heat.
2) Brush the fillets all over with olive oil and season generously with salt and pepper. Lay the fish on the griddle, rounded-side down, and leave it until you can lift the fish without it sticking to the griddle and there are distinct griddle marks, about 5 minutes. (Test it by gently lifting a corner--if it sticks, cook it a bit longer and try again). Carefully turn the fish over and cook until firm to the touch, about another 5 minutes. Remove from the griddle onto 2 plates, and brush with some of the mojo.
3) Garnish with coriander sprigs and serve with the sauce.