Combine 1 cup water and 1 cup sugar in a small saucepan over medium heat and stir until sugar dissolves.
Add 4 tsp dried culinary lavender buds (or 1/4 cup fresh) to the syrup and bring to a gentle simmer.
Remove from heat and steep the lavender in the syrup for 10–15 minutes, covered, to extract fragrance.
Strain the lavender simple syrup through a fine mesh sieve into a bowl, pressing gently to extract flavor; discard solids.
Juice 4–6 lemons to yield about 1 cup (240 ml) fresh lemon juice; strain to remove seeds and pulp if desired.
In a large pitcher, combine the cooled lavender simple syrup and fresh lemon juice.
Add 4–5 cups cold water to the pitcher, tasting and adjusting for sweetness and tartness.
Refrigerate the lemonade for at least 1 hour to chill and let flavors meld.
Fill glasses with ice and pour the chilled lavender lemonade over the ice.
Garnish each glass with a lemon slice and a small sprig of lavender; serve immediately.