The secret to making "ice cream" in a flash is to freeze the fruits beforehand, so they act like ice cubes and help lend body to the puree (besides lending flavor and also color). In this summery version, fresh strawberries and raspberries combine to make a vibrantly hued dessert that's got much less fat, and also a welcome tang thanks to the plain yogurt in the mix, while lemon adds pleasant tartness to balance out the sweetness.