Make the raspberry syrup: In a small saucepan, combine 2 cups raspberries, 1 cup sugar, and 1 cup water. Bring to a gentle simmer over medium heat, stirring until the sugar dissolves. Let it bubble softly for 5–7 minutes, mashing the berries lightly with a spoon.

Strain and cool: Set a fine-mesh strainer over a bowl or measuring cup. Pour the mixture through, pressing on the solids to extract all the juice. Discard the seeds and pulp. Let the syrup cool to room temperature, then refrigerate until chilled.

Juice the lemons: Roll the lemons on the counter to loosen the juice. Cut and squeeze until you have about 1 cup fresh lemon juice. Strain out seeds and excess pulp if you prefer a smoother drink.

Mix the base: In a large pitcher, combine the chilled raspberry syrup and lemon juice. Taste. If it’s too tart, add a splash of water or a little more syrup. If it’s too sweet, add extra lemon juice or water.

Add bubbles right before serving: Fill the pitcher with ice. Pour in 4–5 cups chilled sparkling water or club soda. Stir gently to combine without knocking out the fizz.

Garnish and serve: Add a handful of raspberries and lemon slices to the pitcher. Pour into ice-filled glasses, and tuck in a sprig of mint if you like.