Wash peaches thoroughly but do not peel. Halve, remove pit, and cut each half into eight slices. In a medium bowl, toss peaches with agave and orange juice. Refrigerate for about 2 hours or overnight.

Make sorbet: Combine sugar, water, peppermint, and lemon zest in a medium pot and bring to a boil. Reduce heat to a steady simmer and continue to cook for 5 minutes. Remove from heat and let steep for 20 minutes; then strain through a fine sieve into a bowl, pressing to extract flavors. Stir in lemon juice. Chill until cold, at least 45 minutes. Pour into an ice-cream maker and freeze according to manufacturer’s directions. (Or freeze in an 8x8-inch pan, scraping with a fork every 15 minutes.)

Toss raspberries with a few tablespoons of peach liquid. Serve peaches and raspberries atop scoops of sorbet, drizzled with remaining juice. Garnish with mint leaves.