STEP 1: Cut one lemon into thin slices. Juice half of the other lemon and set aside.
STEP 2: Pat the Mahi-Mahi fillets and season with salt and pepper.
STEP 3: Heat a large cast iron skillet with olive oil over medium-high heat (just until it starts to smoke). Add the Mahi-Mahi fillets and sear until well browned at the bottom (and the sides are cooked halfway up the fillets). Flip and continue to sear (2 to 4 minutes). Transfer to serving plate.
STEP 4: Reduce heat to medium-low. Add lemon juice, garlic, and salt. Scrape the cast iron skillet to get the browned bits from the bottom of the pan mixed into the sauce.
STEP 5: Add lemon slices and stir in the butter one piece at a time.
STEP 6: Remove the cast iron skillet from the heat and stir in the parsley. Taste the sauce and season as needed. Drizzle the sauce over the Mahi-Mahi and serve immediately.