Preheat saute pan over medium high heat (hot pan, cold oil, food won't stick) until just warm and add the olive oil. Let oil heat and place fillets in the pan. Let sides brown for 1 minute or 2, then add onion. Stir until onion starts to become translucent. Add garlic and rosemary. Cook for 1 to 2 minutes until garlic just begins to brown. Flip fillets over to brown. Add olives and stir in. Then, add capers. Stir for 1 to 2 minutes, then, add white wine. Increase heat for a few minutes to reduce wine. Reduce heat to medium and add tomato sauce. Stir to combine all ingredients and let sit for a few minutes. Flip fillets so that both sides are coated with sauce. Fish is done when there is resistance to pressure. (Check also with your seafood market person about recommended cooking times for the fish.) Freshly ground pepper can be added during the cooking, but salting the dish isn't necessary since there is a lot of salt in the olives and capers. Serve with rice, pasta, or couscous. Garnish with a sprig of fresh rosemary.