Don't even bother stirring this super easy summer drink. You want heavy ice cubs to hold down the bits of berry and keep this drink chilled.
Stir sugar and water in heavy small saucepan over medium-low heat until sugar dissolves. Increase heat and bring to a boil. Cool syrup.
Divide strawberries among six 6-to 8-ounce glasses; add 1 tablespoon syrup to each and mash with muddler or handle of wooden spoon. Add lemon slice to each and mash to release flavor. Add several ice cubes to each glass, then fill with Prosecco.