Preheat oven to 375 degrees F. Heat oil in a 10-inch nonstick skillet over medium heat. Cook the sausage, crumbling with the back of a wooden spoon, until no longer pink, about 3 to 4 minutes. Add the scallions, season with 1/4 teaspoon salt and cook, stirring, until the scallions begin to wilt, about 2 to 3 minutes. Add bell pepper, and cook, stirring until wilted but not completely limp, about 8 to 10 minutes.
In a bowl, beat eggs with the milk and remaining salt. Let the bread cubes soak in the egg-milk mixture until moistened, about 2 to 3 minutes. Reduce heat under skillet to medium-low, then pour in eggs and bread and let cook until the eggs begin to set around the edges of the pan, about 2 to 3 minutes.
Sprinkle all over with the grated cheese. Put the skillet in the oven, and bake until frittata is set all the way through and the top is golden, about 18 minutes. Let rest for a few minutes, then run a knife around the edge of the skillet and invert onto a plate or cutting board. Serve in wedges, warm or at room temperature.