• 4 medium peaches, about 1 1/2 pounds, peeled and cut into 1/2-inch-wide slices
• 3/4 cup sugar
• 1/2 cup cider vinegar
• 1/4 teaspoon hot red-pepper flakes
• 1 small red onion, halved lengthwise and sliced crosswise 1/8 inch thick (1 cup)
• 3/4 teaspoon salt

Combine peaches, sugar, vinegar, and hot pepper flakes, and briskly simmer, uncovered, until liquid thickens slightly, 15 to 20 minutes. Stir in onion and salt and simmer for 5 minutes. Relish will continue to thicken as it cools.