In a 12-inch nonstick skillet over medium, heat the oil until barely smoking. Add the sausages, cover and cook, turning occasionally, until the centers reach 160°F, 8 to 10 minutes. Transfer to a cutting board.
In the same skillet over medium, combine the onion and ¼ cup water. Cook, stirring occasionally, until the onion begins to soften, 5 to 7 minutes. Stir in the grapes, then cover and cook, stirring occasionally, until the grapes begin to soften and some have burst, 7 to 8 minutes; press on the grapes with a silicone spatula to help release the juices. Add the vinegar and cook, uncovered and stirring occasionally, until reduced to a light glaze, 3 to 4 minutes. Remove the pan from the heat.
Cut the sausages on the diagonal into 1- to 2-inch pieces and add to the skillet. Cook over medium, stirring occasionally, until heated through, about 2 minutes. Off heat, stir in the parsley. Taste and season with salt and pepper.
