Juice your lemons and measure out one third cup of fresh lemon juice. Set it aside. Having everything measured before you start makes the blending go faster.

Add the frozen raspberries, lemon juice, simple syrup, and cold water to the blender. Add the ice cubes on top.

Blend on high for about thirty to forty-five seconds. Stop and check the texture. It should look thick and uniformly slushy with no large ice chunks remaining. If the blender is struggling, add cold water one tablespoon at a time and pulse briefly between additions. Do not add too much at once or the slush will go watery.

Taste the slush and add more simple syrup if it is too tart for your preference, then pulse once or twice more to mix it in.

Pour immediately into two tall glasses. The slush is thick, so use a wide-mouth glass and a spoon or a thick straw. Garnish with a lemon slice and a fresh raspberry on the rim if you like, and serve right away before it melts.