To make the syrup, add the sugar and water to a small saucepan. Turn the heat to medium-high and gently swirl the pan to mix in the sugar. Heat to a boil and cook for 1 minute. Remove from heat and stir in the lavender.

Let cool completely, at least 1 hour, up to 8 hours. The longer the lavender sits in the syrup, the stronger the flavor. When it reaches your preferred strength of flavor, strain the syrup and discard the lavender.

To make the lemonade, add the syrup, water, and lemon juice to a high-powered blender. Add 2 cups of ice, secure the lid and blend on high. Add more ice until the drink reaches your desired slushie consistency.

Pour into glasses, garnish with lemon slices, and serve right away.