Make the lavender simple syrup: In a small saucepan set over medium heat, combine the sugar and water. Put the lavender in a mesh tea bag or metal tea ball and add to the pan. Stir to ensure no sugar sticks to the bottom of the pan and bring the mixture to a boil. Lower the heat and let simmer until the sugar is completely dissolved. Turn off the heat and let the syrup steep and cool. Discard the lavender, strain if necessary, and transfer to an airtight container or Mason jar.
Wet the rim of a glass and dip in a mixture of dried lavender and sugar. Fill the glass with ice and top with lavender syrup, tequila, lime juice, and a splash of seltzer. Garnish with a sprig of lavender and serve.