Pre-heat oven to 450°F.
On a 13 x 18-inch rimmed baking sheet, toss together onion slices, 2 tablespoons oil, salt and pepper, and spread in an even layer.
Roast for 8 to 12 minutes, until the onions turn translucent and the thinnest pieces take on a pale gold color at the edges.
After 10 minutes, add grapes, seeds and the remaining 2 tablespoons oil to pan with onions, and toss well.
Spread in an even layer and nestle sausage into the mixture.
Roast until sausage has browned, 25 to 30 minutes, flipping sausage and tossing grapes and onions halfway through.
Transfer sausages to a platter or to individual plates.
Add parsley and chives to pan with grapes and onions and gently toss; this warms up and wilts the herbs.
Use a slotted spoon to transfer grapes and onions to plates with the sausages.
Add vinegar to rimmed baking sheet and scrape up any browned bits on the tray.
Drizzle pan juices over the grapes and sausages, taste, and sprinkle more salt or vinegar on top, if desired.
