1. Heat grill pan over moderately high heat until hot but not smoking. Meanwhile, toss scallions with 2 teaspoons oil and season with salt and pepper. Pat salmon dry and coat with remaining 2 teaspoons oil and season with salt and pepper.
2. Cook scallions in grill pan, turning occasionally, until soft and slightly charred, about 6 minutes, then transfer to a cutting board to cool.
3. Add salmon to grill pan and cook, turning over once, until just cooked through, about 12 minutes total. Transfer with a metal spatula to a plate and cool slightly, then gently flake.
4. While salmon is grilling, finely chop scallions and stir into mayonnaise with celery, lemon juice, and mint. Season with salt and pepper.
5. Toast bread and spread lightly with some of scallion mayonnaise. Divide salmon among toasts and top with dollops of scallion mayonnaise and pieces of frisee.

Yield: 4 servings