For the sausage: If raw, cook in a 400°F oven to an internal temperature of 160°F using a thermometer. If already cooked: Place in a 400 degree oven until warmed through or slice and brown in a nonstick pan on medium heat.
Heat an oven safe nonstick pan on medium heat. Once hot, add the oil. Saute the potato until it starts to become tender. Season with salt and pepper.
Stir the eggs and scallion into the pan. Stir the eggs around to distribute the ingredients evenly. Evenly distribute the goat cheese. Place the pan in the oven until the eggs are firm throughout, about 8 to 10 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, sausage, and plum.