Season the fish portions with salt and pepper. Add butter to a frying pan, over medium high heat. When melted, add garlic and cook it for 1-2 minutes.
Add mushrooms, salt and pepper, and sautée for 5 minutes.
On the same skillet, make room for the Mahi portions. Sear them 2-3 minutes on each side. Remove the fish from the skillet and reserve.
Prepare the sauce in the same skillet.
Add butter on medium heat. When it starts to brown, add capers, parsley and lemon juice. Stir for 1 minute and serve.
Serve the sweet potato purée on plate, place the fish on top and spoon over the sauce. Serve with lemon wedges if you like.