Over a hot grill, toast the jalapeno until lightly blistered on the outside and fragrant. Slice into 1/4-inch-thick wheels.
In a cocktail shaker, add 2 slices roasted jalapeno, ice and the Homemade Sour Mix. Muddle, then add the tequila and the juice of 1 lime wedge. Shake. Rim a margarita glass with the wedge of a lime and salt, if using. Serve over ice. Garnish with a lime wheel.
Homemade Sour Mix:
Combine the sugar and 1 1/2 cups water in a saucepan. Bring to a simmer until the sugar dissolves. Remove from the heat and cool. Add the lemon and lime juice. Chill until ready to use.