Instructions

Make the tapenade

• Process the olives, parsley, anchovy, capers, garlic, and lemon zest in a food processor until finely chopped. With the motor running, gradually add the oil and process until combined. Reserve 1/4 cup for the fish; refrigerate the remainder in an airtight container for another use.

Cook the ragù and the fish

• Melt the butter in a 12-inch nonstick skillet over medium heat. Cook, swirling occasionally, until fragrant and the milk solids are golden brown, about 2 minutes. Stir in the tomatoes, raisins, lemon juice, and pine nuts. Add the spinach and stir until just wilted, about 2 minutes, then stir in the Parmigiano, 1/2 tsp. salt, and 1/4 tsp. pepper. Transfer to a bowl and cover with foil to keep warm. Wipe out the skillet.
• Pat the tuna dry and lightly season with salt and pepper. Heat the olive oil in the skillet over medium-high heat until shimmering hot. Add the tuna and cook, undisturbed, until the bottom 1/8 inch is whitened for medium rare (1/4 inch for medium), about 1 minute. Flip, spread the reserved tapenade over the top, and cook until the bottom 1/8 to 1/4 inch is whitened but the center is still red, about 1 minute more. Serve with the ragù.