How to Make It
Preheat the oven to 350°. Butter a 2-quart baking dish. In a large bowl, toss the peaches with the sugar and lemon juice. Cover and let stand at room temperature for about 30 minutes, stirring occasionally, until the sugar dissolves and the peaches have released some of their juices. Drain the peaches in a colander set over a small saucepan, then return them to the bowl.
Add 1/4 cup of water and the cornstarch to the peach juices and bring to a simmer. Cook, whisking constantly, until thickened and translucent, about 1 minute. Add the thickened juices and the salt to the peaches and toss to coat. Scrape into the prepared baking dish.
In a medium bowl, whisk the flour with the oats, the 3 sugars, the salt and cinnamon. In a small saucepan, cook the butter over moderately low heat, stirring, until deep golden and nutty-smelling, about 8 minutes. Scrape the butter and any browned bits at the bottom of the pan into the flour mixture and stir until well combined.
Press the topping into small clumps and scatter over the peaches. Bake for 50 to 60 minutes, until the crisp is golden and bubbling. Transfer to a rack and let stand for 15 minutes before serving with vanilla ice cream.