Preheat the oven to 350°F. Put the hazelnuts on a sided baking sheet and bake until lightly toasted and fragrant, about 10 minutes. Wrap the hazelnuts in a kitchen towel and rub them together to remove the skins. Finely chop the hazelnuts and put in a small bowl.

Put the peppercorns in a sealable plastic bag and crush with a rolling pin or mallet. In a small bowl, combine the crushed pepper, lemon zest, and salt; mix well. Rub half of the mixture over both sides of the tuna; stir the remaining half into the hazelnuts and set aside.

For the couscous: In a medium saucepan, bring the water, butter, and salt to a boil. Remove from the heat, add the couscous, and cover. Let stand for 5 minutes. Fluff with a fork and stir in the cilantro and lemon juice. Arrange in a mound on each of 6 small plates.

In a medium nonstick skillet, heat the oil over high heat until it shimmers but is not yet smoking. Add the tuna and cook 1-2 minutes. Turn and sear the other side, about 1 minute. (It will be rare on the inside.) Transfer the tuna to a cutting board and cut into thin slices. Fan a few slices over each mound of couscous, sprinkle equally with the hazelnut mixture, and serve with lemon wedges.