In a small saucepan, stir together the cherries, apricots, sugar, lemon juice, and salt. Simmer for about 3-4 minutes, or until the fruit is bubbly. You may have to use a wooden spoon to break them up a bit.
Lower the heat and whisk together the cornstarch and water. Stir the slurry into the fruit, stirring constantly, until it is a thick, jam-like consistency.
Remove from the heat, transfer to a bowl and refrigerate for at least 30 minutes to cool completely.
Preheat the oven to 400°F. Lightly brush the tops of the tarts with the egg wash. Use a fork to poke small holes in the top of the dough to allow steam to escape.
Gather the scraps of dough and re-roll to cut more rectangles if necessary.
Bake for 15 minutes or until golden brown. Remove from the oven and let it cool a little before glazing.
Whisk all of the glaze ingredients together in a medium bowl until it reaches a spreading consistency. If the glaze seems too thick you can add another teaspoon or two of milk. Use a spoon to glaze each tart and top with sprinkles, if desired.