Line baking sheet with parchment paper and lay out your fish. Tuck the thin tail end under to make a thicker layer so the fish will cook evenly.
Arrange the lemon slices on and around the fish, along with the olives, tomatoes, peppers, capers, and onions. Drizzle olive oil over all, and sprinkle with salt and pepper. Dot with chunks of butter, if using.
Bake for about 20-30 minutes, or until the fish is done through and flakes easily with a fork. The exact cooking time will depend on the size and thickness of your fish, and if your fish is especially thick, or heavier, it will take longer.
Garnish with fresh herbs before serving.
Video
Notes
Make it with halibut, tilapia, trout, swordfish, tuna steaks, or even shrimp.
Make it in parchment ~ wrap the whole thing up in parchment paper and pop it in the oven.
Make it with fennel ~ fennel is another great Mediterranean ingredient, lay the fish on a bed of very thinly sliced fennel, and use the fronds as a garnish.