Meanwhile, heat the oil in a large saute pan over medium heat, tilting the pan to coat. Add the onion and garlic and cook, stirring, until softened, 3 to 4 minutes. Add the sausage and cook until just beginning to brown, 3 to 4 minutes.
When the black-eyed peas are tender, stir in the sausage mixture and reduce the heat to a simmer. Let the black eyed peas simmer until the liquid is completely cooked down and thickened, 2 1/2 to 3 hours.