Place the peaches in a large mixing bowl and sprinkle with granulated sugar. Toss gently to combine. Cover the bowl with plastic wrap and set aside at room temperature for 1 hour.
While the peaches are macerating, prepare the topping. In a medium mixing bowl combine the flour, oats, sugars, almonds, cinnamon and salt. Combine the melted butter and vanilla and pour over the oat mixture. Stir to combine pressing the topping together to form large chunks. Refrigerate until needed.
Preheat oven to 350°F. Lightly grease or butter a 1 ½ to 2 quart baking dish or a 9x9-inch square baking pan.
Drain the peach juice into a medium saucepan. Set the peaches aside. Whisk the cornstarch, cinnamon and salt into the juice. Cook over medium heat whisking constantly until the syrup boils. Simmer for 2 to 3 minutes or until thickened.
Remove from the heat and stir in the butter, lemon juice, and vanilla extract. Gently fold in the peaches and any accumulated juices. Once combined pour the peach filling into the prepared baking dish.
Crumble the chilled topping mixture over the peaches leaving some pieces in large clumps, about the size of a quarter. Sprinkle with coarse sugar if desired and additional sliced almonds. Bake on a rimmed baking sheet for 30 to 40 minutes until well browned and bubbly all over. Tent with foil if needed to prevent over-browning.
Remove from the oven and cool slightly before serving. Top with ice cream, sweetened whipped cream, or a drizzle of heavy cream if desired.
Notes
Nutrition information is calculated using an ingredient database and should be considered an estimate. When multiple ingredient alternatives are given, the first listed is calculated for nutrition. Garnishes and optional ingredients are not included.