Get the pan to medium. Add olive oil and butter. Season and pan sear whitefish (winter trout pictured) with rosemary, about 2 & 1/2 minutes per side. Move to a warm plate. Add shallot or onion to pan, sauté. Deglaze with white wine, lemon juice, and capers. Reduce to pan sauce. Season to taste.
Serve with roasted potatoes and broccoli in winter.