Combine the butter, 1 T lime juice and ¼ tsp. each of salt and pepper in a medium saucepan. Cook over low heat, stirring until the butter melts. Turn burner to the lowest setting to keep the mix warm.
STEP 2: Heat a 12” skillet over medium-high heat.
STEP 3: While the pan is heating, whisk together 1 T of olive oil with the remaining lime juice and a pinch of salt and pepper. Drizzle over the spinach and toss gently. Add the sliced onions and avocado; toss gently so as not to mash the avocado. Set aside.
STEP 4: Season the mahi-mahi on each side with salt and pepper. Add to the hot pan and cook for 3 minutes on each side, or until the fish is just opaque in the middle.
STEP 5: Gently break the fish into large chunks and arrange on top of the salad.
STEP 6: Add the fish pan juices to the butter lime mix, stir and drizzle over the salad.
Serve with freshly cracked pepper and enjoy!