Lay the 4 tilapia fillets in the bottom of the slow cooker.
Top the fillets with the 2 tablespoons butter and the slices from the 1 lemon, then sprinkle with the ½ teaspoon salt and a pinch of black pepper.
Cover and cook on High for 1 hour 30 minutes, until the fish is opaque and flakes easily with a fork.
Tip: Tilapia is very lean and overcooks fast, so start checking early and pull it the moment it flakes.
Oven method
Place the seasoned fillets on a lined baking sheet, topped with the butter and lemon.
Bake at 400°F for about 15 minutes, until the fish flakes easily.
Notes
Make ahead Season the fillets and slice the lemon ahead so you can start the slow cooker in seconds.Store leftovers in an airtight container in the fridge for up to 3 days.Freeze Cooked tilapia can be frozen for up to 2 months, though the texture softens; it is best fresh.Reheat gently in a 300°F oven or the microwave on low power, just until warm.Substitutions Cod, catfish, or any mild white fish work with the same butter-and-lemon method.Pro tip Thaw frozen fish fully and pat it very dry before seasoning so the dish does not turn watery