Preheat the Oven: Set a rack near the top of your oven and preheat it to 300°F.
Prepare the Scallions: Slice each scallion lengthwise, then cut into pieces about 3 inches long.
Cook the Scallions: Heat 2 tablespoons of olive oil in a nonstick, oven-safe skillet over medium-high heat. Add the scallions and cook until softened and lightly browned in places, about 3 to 5 minutes. Sprinkle with salt to season.
Mix the Eggs: In a large mixing bowl, beat the eggs. Stir in ½ teaspoon of salt and add black pepper to taste.
Bake the Frittata: Pour the egg mixture into the skillet with the cooked scallions. Gently stir to combine, then transfer the skillet to the oven. Bake until the eggs are just set, which should take 5 to 8 minutes.
Cut the Frittata: Remove the skillet from the oven and slide the frittata onto a flat surface. Slice it into thin ribbons about ¼ inch wide.
Prepare the Toast: Take the toasted bread slices and rub each one with the cut side of a halved garlic clove. Drizzle olive oil over the surface of the bread.
Assemble the Crostini: Place the frittata ribbons on top of the prepared toast slices. Drizzle with a little lemon juice and additional olive oil if desired. Sprinkle the sliced olives and fresh herbs over the top to finish.