Prepare the Peaches: In a cocktail shaker, muddle about three-quarters of the sliced peaches with 10 mint leaves to release their flavors. It's fine if the peach slices break up; this releases more juice.

Mix the Drink: Add the bourbon, peach liqueur, and simple syrup to the shaker. Fill the shaker halfway with crushed ice, then shake well until the mixture is chilled.

Assemble the Julep: Fill a julep cup or a similar glass with crushed ice. Strain the mixture from the shaker into the glass over the ice.

Garnish and Serve: Garnish with the remaining peach slices and fresh mint leaves. You can slap the mint leaves between your hands before placing them in the drink to intensify their aroma.

Enjoy: Stir the mixture gently in the glass before drinking to mix the flavors together.