Set oven to 375°. Line baking sheet with parchment paper. Slice lemons and place 3 slices under each frozen fillet on sheet. Drizzle on melted butter. Top the tilapia fillets with chili pepper flakes, salt, pepper, and parsley.
Bake approx. 20-22 minutes or until tilapia is cooked thoroughly into the center and fork tender. Timing will vary depending on how thick yours are, check at 18 minutes if they're on the smaller side.