Slice half the lemon thinly. Juice the remaining half to yield about 1 1/2 tablespoons juice; set both aside.

Pat mahi mahi dry. Season all over with 1 teaspoon salt and a few grinds of pepper.

Heat oil in a large cast-iron or nonstick skillet over medium-high until just starting to smoke. Sear mahi mahi undisturbed about 4 minutes, then flip and cook 2-4 minutes more until just cooked through. Transfer to a serving platter.

Reduce heat to medium-low. Add lemon juice, garlic and remaining 1/4 teaspoon salt, scraping up browned bits.

Add lemon slices, then stir in butter one piece at a time, waiting until each is almost melted before adding the next.

Remove from heat, stir in parsley, taste and adjust salt and pepper. Pour sauce over mahi mahi and serve immediately.