With an electric mixer set to medium whisk together the egg whites, cream of tartar and salt for 3 minutes. Let the mixer keep running and add in the sugar, a spoonful at a time. Once the meringue is thick and can form stiff peaks you're done (about 6 minutes total)!
Cover a baking sheet with parchment paper. Spoon on big dollops of meringue and press the centers to form little bowls (to hold the jam later). Bake at 250°F for 1 hour and 20 minutes. Halfway through baking be sure to rotate the pan(s) so that they bake evenly.
To make the jam combine the blueberries, sugar, corn syrup, lemon juice + zest in a small pan. Cook over medium heat (so it just reaches the boiling point) for 6-8 minutes. Remove from heat, the jam will thicken as it cools. Top the Pavlovas with warm jam and a mint sprig.