GRILLED TUNA WITH POTATO-TOMATO GRATIN AND ROUILLE

Steps:

• Season the fish with the lemon zest, chile, thyme, and parsley. Cover, and refrigerate at least 4 hours.
• Light the grill 30 to 40 minutes before cooking, and remove the fish from the refrigerator to come to room temperature.
• When the coals are broken down, red, and glowing, brush the fish with olive oil, and season on both sides with fleur de sel and cracked black pepper. Grill the fish 2 to 3 minutes per side, rotating it once or twice. The tuna should be well seared but still rare.
• Scatter the arugula onto a large platter and arrange the tuna on top. Squeeze some lemon juice over the fish, and then drizzle the fish with the super-good olive oil. Dollop each piece of fish with rouille, and serve the potato-tomato gratin and the rest of the rouille on the side.
• Char the pepper on a medium-hot grill, on the burner of a gas stove, or in the broiler. Place the pepper in a small paper bag and close tightly. Let steam 15 minutes.
• Toast the saffron in a small pan over medium heat until it just dries and becomes brittle. Be careful not to burn it.
• Place the egg yolk in a stainless steel bowl. Begin to whisk in the grapeseed oil drop by drop, as slowly as you can bear. Once the mayonnaise has thickened and emulsified, you can add the remaining oils in a slow steady stream, whisking all the time. If the mixture gets too thick, add a drop or two of water.
• Using a mortar and pestle, pound the saffron, garlic, and 1/2 teaspoon salt. Scrape every little bit of the mixture into the mayonnaise with a rubber spatula, and whisk to combine.
• Peel the charred and steamed pepper carefully over a strainer. Do not run the pepper under water or you will lose some of the delicious juices. Remove the stem, seeds, and membranes.
• Purée the pepper in a food processor until smooth. Fold it into the mayonnaise, and season with a healthy squeeze of lemon juice, a pinch of cayenne, and a pinch of black pepper. Taste for balance and seasoning.
• Heat a large sauté pan or Dutch oven over high heat for 2 minutes. Swirl in 3 tablespoons olive oil, and add the onions, 1 teaspoon thyme, 1 teaspoon salt, and some pepper. Cook 6 minutes, stirring often, and then turn the heat down to medium. Add the butter, and cook 15 minutes, stirring and scraping with a wooden spoon, until the onions start to caramelize. Turn the heat down to low, and continue cooking for about 10 minutes, stirring often, until the onions are a deep golden brown. Remove the pan from the heat and set aside.
• Preheat the oven to 350°F.
• Use a mandoline to slice the potatoes into 1/8-inch-thick rounds. Toss them with the cream, 1 teaspoon thyme, 1 teaspoon salt, and some freshly ground black pepper.
• Cut the tomatoes into 1/4-inch-thick slices, arrange them on a plate, and season them with 1 teaspoon salt and some pepper.
• Place half the caramelized onions in an even layer in a 9-by-9-inch (or equivalent) gratin or baking dish. Arrange one layer of alternating potatoes and tomatoes on top of the onion layer. Drizzle with 2 tablespoons of the cream from the potatoes and 1 tablespoon olive oil. Season with 1/4 teaspoon salt, a healthy pinch of pepper, 1/2 teaspoon thyme, and half the basil.
• Scatter the rest of the caramelized onions over the potatoes and tomatoes.
• Arrange another layer of potatoes and tomatoes on top; make this layer pretty, because it will be the top of your gratin. Pour the remaining cream (from the potatoes) and remaining tablespoon olive oil over the potatoes and tomatoes. Season with 1/4 teaspoon salt, a pinch of pepper, the remaining 1/2 teaspoon thyme, and the remaining basil. Press the vegetables down with your fingers. The cream and oil will come up through the layers and coat the vegetables evenly.
• Cover the baking dish tightly with aluminum foil. Bake about 2 hours, until the potatoes are tender when pierced. Remove the gratin from the oven and uncover it, being careful of the steam.
• Turn the oven up to 450°F and return the gratin, uncovered, to the oven. Cook another 25 to 30 minutes, until the juices have thickened and the top is nice and golden brown (as in "gratinéed").
• Season the tuna with the herbs and lemon zest in the morning. The gratin is easy to assemble but takes quite a while in the oven so start early. You can make it in advance and reheat just before serving. The rouille can be made a few hours before serving.