In a large frypan heat olive oil and cook onions, mushrooms and garlic over medium heat until onions are transparent.
Cut the mahi mahi into 8cm fillets. Place the fillets over the onions, mushrooms and garlic. Salt and pepper the first side of the fillets to taste. Add white cooking wine and lemon juice.
Cover and cook 4 to 5 minutes. Turn the fillets over and salt and pepper the second side to taste. Cook 4 to 5 minutes or until fish flakes easily.
Remove only the fish to a heated plate and keep warm until sauce is ready.
In the same frypan with all the onions, mushrooms, garlic and cooking wine, raise heat to medium/high. Bring to a boil. Dissolve cornflour in the 2 tablespoons of water; stir into frypan. Stir the sauce continuously until it thickens to desired consistency.
Pour sauce over mahi mahi fillets. Serve immediately.