Preheat oven to 425°. In a 12-inch cast-iron skillet, cook sausage over medium-high heat, stirring occasionally, until browned, about 5 minutes. Using a slotted spoon, remove sausage from pan. Add oil to skillet; preheat in oven 8 to 10 minutes.
In a large bowl, stir together sausage and remaining ingredients. Carefully remove skillet from oven; pour batter into hot oil in skillet. (Do not stir.)
Bake until a wooden pick inserted in center comes out clean, 20 to 25 minutes. (Cover with foil to prevent excess browning, if necessary.)