Oysters with Champagne-Vinegar Mignonette

For mignonnette
2 teaspoons Champagne vinegar
1 1/2 teaspoons finely chopped shallot
Pinch of coarsely ground black pepper
Pinch of sugar
1 teaspoon finely chopped fresh flat-leaf parsley

For oysters
1 1/2 cups kosher or other coarse salt
1/2 dozen small oysters,, shells scrubbed well and oysters left on the half
shell their liquor reserved and oysters picked over for shell fragments
1/2 tablespoon unsalted butter, cut into 6 pieces
1 small cluster Champagne table grapes or 2 finely diced seedless red grapes

Make mignonnette:
Stir together vinegar, shallot, pepper, and sugar and let stand 30 minutes.

Prepare oysters:
Preheat broiler.

Spread 3/4 cup salt in an 8- to 10-inch flameproof shallow baking dish or pan. Arrange oysters on their shells in salt, then top each with a piece of butter.

Broil 4 to 6 inches from heat until butter is melted and sizzling and edges of oysters are beginning to curl, 1 to 2 minutes.

Stir parsley into mignonnette. Divide remaining 3/4 cup salt between 2 plates and arrange 3 oysters on each. Spoon 1/4 teaspoon mignonnette over each oyster and sprinkle oysters with grapes. Serve warm.

Cooks' note:
Mignonnette, without parsley, can be made 1 day ahead and chilled, covered.