For the chicken sausage: cook in the oven to an internal temperature of 160°F using a thermometer.
Heat an oven safe nonstick pan on medium heat. Once hot, add the oil and butter. Saute the onion until golden brown. Remove the pan from the heat.
Stir the eggs, scallions, and cheese into the pan. Stir the eggs around in short circular motions to distribute the ingredients evenly and temper the eggs to the hot pan. Place the pan in the oven until the eggs are firm throughout, about 8 minutes.
To release the frittata from the pan, run a spatula around the edge, then underneath, until you can slide or lift it out onto a cutting board or serving plate.
Cut into wedges. Serve with toast, chicken sausage, and pineapple.