Peel peaches (I mince up the peels for our chickens), then halve and pit each. Cut each half into 4 slices. Gently toss peaches and sugar together in a large bowl; let stand for 30 minutes, tossing several times. Drain peaches in colander set over large bowl. Whisk 1/4 cup of the drained juice (I rarely have more 1/4 cup juice, but if you do, save it for a morning smoothie), cornstarch, lemon juice, and salt together in a small bowl. Toss peach juice mixture with peach slices and transfer to 8-inch square glass baking dish. Bake until peaches begin to bubble around edges, about 10 minutes.