Slice the peppers into thick strips and reserve them in a bowl with any juices. Light a charcoal or gas grill or heat a cast-iron skillet to high. Lightly coat the fresh tuna on all sides with a little of the olive oil, season with salt, pepper, and very little cayenne, and grill it over a hot flame to a medium doneness (just a trace of pink in the center when sliced). Set aside.
Peel the potatoes, cut them into quarters, and put them in a medium pot with cold water to cover. Add 1 Tbs. salt and bring to a boil over high heat. As soon as the potatoes come to a boil, reduce the heat and simmer until they’re just tender, 10 to 12 minutes. Drain, run under cold water, and cut each quarter in half. Reserve.
Put the eggs in a small pot, cover with cold water, and bring to a boil over high heat. Once boiling, reduce the heat to a hard simmer and cook for 8 minutes. Remove from the heat, drain, and chill in ice water. Once cooled, peel the eggs and reserve.
In a sauté pan, heat the rest of the oil with the garlic over low heat until the garlic is golden, 10 to 12 minutes. Let cool for 10 minutes, strain out the garlic, and reserve the oil and garlic separately.
Stir the lemon juice into the garlic-flavored oil and add 12 tsp. salt, pepper, and a pinch of cayenne; set aside.
Slice the tuna into bite-size pieces and put in a bowl. Add the potatoes, olives, and celery and mix gently. Add 3 to 4 Tbs. of the garlic dressing and toss. Season with salt, pepper, and a pinch of cayenne. Toss 3 Tbs. of the dressing with the pepper slices, and another 3 to 4 Tbs. of the dressing with the arugula. Reserve any extra dressing to drizzle on the completed salad, if you like.
To serve the salad, arrange the greens on a shallow platter, mound the tuna-potato mixture in the middle, and arrange the roasted red pepper slices around that. Cut the hard-boiled eggs into wedges and arrange them around the perimeter. Sprinkle the garlic slivers and the sliced scallions over all.