Make a quick simple syrup: In a small saucepan, combine sugar and 1/2 cup water. Heat over medium, stirring, until sugar dissolves completely. Remove from heat and let cool for 5 minutes. (This prevents grainy texture.) 2. In a blender, add frozen strawberries, lemon juice, cooled simple syrup, and ice cubes. 3. Blend on high for 30-45 seconds, until the mixture is thick and slushy. Stop and scrape down sides if needed. 4. Taste and adjust: For more tartness, add a squeeze of lemon. For sweeter, add a teaspoon more sugar and blend briefly. 5. Pour into two glasses and serve immediately with a straw.