Preparation Cut an X in bottom of each peach, then blanch in a medium saucepan of boiling water 10 seconds. Transfer with a slotted spoon to a bowl of ice and cold water and cool. Peel peaches and coarsely chop. Cook onion, jalapeño, and a pinch of kosher salt in oil in a heavy medium saucepan over medium heat, stirring occasionally, until softened, 8 to 10 minutesutes. Add peaches and remaining ingredients and simmer, uncovered, stirring occasionally, until peaches are very tender, about 30 minutesutes. Purée in a blender (use caution when blending hot liquids). Cooks' note:Sauce can be made 3 days ahead and chilled, uncovered, until cool, then covered.
Preparation Cut an X in bottom of each peach, then blanch in a medium saucepan of boiling water 10 seconds. Transfer with a slotted spoon to a bowl of ice and cold water and cool. Peel peaches and coarsely chop. Cook onion, jalapeño, and a pinch of kosher salt in oil in a heavy medium saucepan over medium heat, stirring occasionally, until softened, 8 to 10 minutesutes. Add peaches and remaining ingredients and simmer, uncovered, stirring occasionally, until peaches are very tender, about 30 minutesutes. Purée in a blender (use caution when blending hot liquids). Cooks' note:Sauce can be made 3 days ahead and chilled, uncovered, until cool, then covered.