Make the peach syrup: In a small saucepan, combine the peaches, honey, and water. Bring to a simmer over medium heat and cook for 10 minutes.
Mash the softened peaches with a spoon or muddler to release all their juice, then strain through a fine mesh strainer. Let cool completely.
Rim your glass with salt and fill it with ice. Set aside.
In a cocktail shaker, muddle the jalapeño slices with the peach syrup for about 10 seconds.
Add ice, then pour in the tequila, Cointreau, and lime juice.
Shake for 10 seconds until well chilled.
Strain into your prepared glass over fresh ice. Top with a splash of sparkling water if you like it bubbly.
Garnish with a peach slice, a slice of jalapeño, and a lime wedge. Stir and enjoy!