Adjust oven rack to lower-middle position and heat oven to 425 degrees. Generously spray middle 6 cups of 12-cup nonstick muffin tin with olive oil spray.
Heat oil in 10-inch nonstick skillet over medium heat until shimmering. Add sausage, bell pepper, and scallion whites and cook, breaking up sausage with wooden spoon, until sausage is no longer pink and pepper is softened, about 4 minutes. Transfer to medium bowl and let cool slightly.
Beat eggs, cream, and pepper in separate bowl with fork until thoroughly combined and mixture is pure yellow; do not overbeat. Stir cheddar and scallion greens into sausage mixture. Divide sausage mixture evenly among prepared muffin cups. Using ladle, evenly distribute egg mixture over filling in cups. Bake until frittatas are lightly puffed and set in centers, 12 to 14 minutes.
Transfer muffin tin to wire rack and let frittatas cool for 10 minutes. Run plastic knife around edges of frittatas, if necessary, to loosen from muffin tin, then gently remove. Serve.