Preheat oven to 400° with oven rack in the middle position.
Spray cooking spray onto a 8x8 inch baking pan.
Finely chop white and green parts of scallions and reserve separately.
Heat oil in a large skillet over a moderately high heat. Cook onions, garlic, white parts of scallions, 1/2 teaspoon salt and 1/8 teaspoon pepper, stirring occasionally (about 5 minutes).
Add sausage, stirring occasionally, until just brown, about 4 minutes.
Reduce heat to moderate, stir in potatoes and cover skillet. Cook until potatoes are tender, stirring occasionally, about 10 minutes. Place in a bowl and cool slightly.
In a bowl, whisk together chilies, eggs, sour cream, cheese, scallion greens, and remaining 1/4 teaspoon salt and 1/8 teaspoon pepper, then pour into baking pan.
Sprinkle potato mixture over eggs. Bake until tortilla is set, about 20 minutes.
Cool in pan on rack to room temperature. Trim outer edges and cut the tortilla in small squares and put on serving platter over a bed of lettuce. Refrigerate until ready to use. Serve at room temperature.