In a nonreactive bowl toss tuna with juice and zest of 3 of the lemons, cracked peppercorns; season with salt and white pepper.
Make lemon aioli: in a small bowl, combine mayonnaise with garlic and juice and zest of remaining 2 lemons; season to taste with salt and pepper. If necessary, thin with 1 to 2 tablespoons water, to consistency of thick cream or yogurt.
When ready to serve, arrange lettuces in a bed on serving platter. Heat a wok over high heat until nearly smoking. Add oil and swirl around wok. Drain tuna and add to wok tossing constantly until well-seared on all sides, 1 to 2 minutes in all. Using a slotted spoon, transfer tuna to serving platter and set on top of greens. Drizzle with aioli and garnish with parsley. Tuck croutons around edges of plate and serve immediately.
Yield: 4 appetizer servings