Heat oven to 425 degrees. On a large, heavy (13-by-18-inch) rimmed baking sheet, toss together onion slices, 2 tablespoons oil, salt, and pepper, and spread in an even layer; do not spread too thinly. it’s okay to have a lot of space around the edges. Roast for 8 to 12 minutes, until the onions turn translucent and the thinnest pieces take on a pale gold color at the edges.

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. If at the halfway point it looks as if the sausages are a little pale and/or the onion mixture is getting a bit dark, move the sausages towards the edge of the onions and grapes where they’ll brown better and protect the onions from the heat.

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). Sprinkle with the herbs and vinegar and toss the sausage grape mixture gently (do not break open the grapes) with a spatula, scraping any browned bits up. Taste and adjust for salt and/or vinegar. Add grapes and onions to the sausages and drizzle any juices over.

Notes:

• Poke the sausages multiple times, about 10 times on all four sides with a fork.
• With Italian sausage, try Michael Chiarello’s Fennel Spice Rub. Note that it has salt in.