1 1/2 teaspoons peppercorns
1 teaspoon fennel seeds
One 8-ounce piece of center-cut tuna loin, about 3 by 3 by 4 inches kosher salt
2 teaspoons olive oil
2 celery stalks, cut lengthwise in half and sliced 1/4 inch thick, leaves reserved for garnish
1 cup cooked butter peas or drained and rinsed canned chickpeas
1 red bell pepper, roasted, peeled, seeded, and cut into 1/2-inch dice
One 1- to 2-ounce black truffle or 3 large button mushrooms, trimmed, cleaned, and cut into julienne strips
1 shallot, finely minced
grated zest and juice of 1 lemon
freshly ground black pepper
2 tablespoons extra virgin olive oil
1 1/2 teaspoons red wine vinegar
1 1/2 teaspoons black truffle oil or more if desired
Toast the peppercorns and fennel seeds in a small skillet over medium heat just until fragrant, about 3 minutes. Transfer to a mortar and crush with the pestle, or crush under a heavy skillet. Spread the peppercorns and fennel seeds on a small plate. Roll the tuna "brick" in the spices and press with the heel of your hand so they adhere. Season the tuna with salt.
Heat a small heavy sauté pan over high heat. Add the olive oil and sear the tuna on all sides, 3 to 4 minutes total; it will still be very rare inside. Transfer the tuna to a rack set over a plate to rest.
To prepare the salad, combine the celery, peas, roasted pepper, truffle, shallot, and lemon zest in a medium bowl and season with salt and pepper. Toss with the extra virgin olive oil, vinegar, lemon juice, and truffle oil.
Slice the tuna crosswise into 1/8-to 1/4-inch-thick slices and arrange like a sunburst in a circle on each of four serving plates. Place the salad in the center and garnish with the reserved celery leaves. Drizzle the tuna with any remaining dressing from the bowl, or a little extra truffle oil, if desired.