Preheat a grill to medium high. Grill the peaches cut-side down until marked, 2 to 3 minutes.
Combine the tequila, Cointreau, lime juice and all but 2 grilled peach halves in a blender. Blend on high for 1 to 2 minutes until smooth.
Slice the remaining grilled peach halves. Divide the drink among ice-filled glasses and garnish with the sliced peaches.
Photograph by David Malosh