Make the lavender syrup
Combine 2 cups water and ½ cup sugar in a saucepan over medium heat. Stir until the sugar dissolves.
Turn off the heat and stir in ¼ cup agave syrup and 3 tablespoons dried lavender.
Let the mixture steep for 2 hours at room temperature (or 30–45 minutes if you want a lighter floral note). Press the lavender gently when you strain to extract flavor.
Build the lemonade base
Strain the syrup through a fine mesh sieve into a pitcher. Add 2 cups freshly squeezed lemon juice and the remaining 4 cups cold water. Stir well. Taste and adjust: more water for milder, more syrup for sweeter.
Assemble the spritz
Fill glasses with ice. Add 1/2 to 3/4 cup lemonade per glass (depending on how strong you want it). Top with sparkling water (for a mocktail) or vegan prosecco (for a cocktail). Stir gently.
Garnish with lemon slices and a fresh lavender sprig. If you want color pop, add a teeny drop of blue/purple food coloring to the pitcher and stir.
Serving ratio (recommended):
Mocktail: 3 parts lemonade : 2 parts sparkling water
Cocktail: 3 parts lemonade : 1 part prosecco : 1 part sparkling water