3 tbsp. olive oil
4 anchovy fillets
2 onions, thinly sliced
2 large cloves garlic, chopped
Salt and pepper
¼ cup pitted oil-cured black olives, chopped
4 tuna steaks (1/2 inch thick), dark-red parts (bloodline) removed
Salt and pepper
2 tsp. herbes de Provence
Cooking spray or a drizzle of EVOO
1 lemon, halved
Butter, at room temperature, for the bread
8 thin slices good-quality white bread
Dijon mustard (I like Maille)
3 to 4 tbsp. chopped cornichons
8 slices Comté, Swiss, or Fontina cheese, from the deli counter
In a medium skillet, heat the oil, three turns of the pan, over medium. Add the anchovies. Cook, stirring and breaking up with a spoon, until they melt into the oil, about 2 minutes. Add the onions and garlic; season with salt and pepper. Cook, stirring often, until the vegetables soften, about 10 minutes. Add 1/2 cup water. Raise heat a touch. Cook until the onions are light to medium golden and very soft, about 10 minutes. Add the olives. Remove from heat.
Meanwhile, heat a large cast-iron skillet over medium-high. Season the tuna with salt, pepper, and the herbes de Provence. Spray the skillet with cooking spray. Cook the tuna for 2 minutes per side for medium or about 3 minutes per side for well-done. Squeeze the lemon over the tuna. Transfer to a plate. Wipe out the skillet; heat over medium-low.
Lightly butter 1 side of each bread slice. Lightly spread the Dijon on the unbuttered sides of 4 slices. Build the sandwiches by topping the mustard sides of the bread with the cornichons, half of the cheese, the tuna, onions, and the remaining cheese. Top with the remaining bread (buttered-side out). Grill until deep golden, about 3 minutes per side. Cut the sandwiches corner to corner.