Step 1

Arrange a rack in the middle of the oven and heat the oven to 375°F. Coat a 2-quart or 11x7-inch baking dish with butter.

Step 2

Make the crisp topping: Cut 12 tablespoons unsalted butter into tablespoon-sized pieces and place in a large microwave-safe bowl. Microwave in 20-second bursts until mostly melted, 40 to 50 seconds total (it will melt completely as it sits). (Alternatively, melt the butter in a small saucepan over medium heat and transfer to a large bowl.) Set aside to cool slightly.

Step 3

Finely grate the zest of 1 medium lemon into the bowl (about 1 tablespoon); reserve the zested lemon for the filling. Add 1 teaspoon vanilla extract, 1 1/2 cups old-fashioned rolled oats, 3/4 cup all-purpose flour or gluten-free all-purpose flour blend, 1/2 cup sliced almonds if using, 1/2 cup packed light brown sugar, 3/4 teaspoon kosher salt, and 3/4 teaspoon ground cinnamon to the bowl and stir to combine. Refrigerate for at least 5 minutes or until ready to use, the topping will harden when chilled.

Step 4

Make the filling: Halve and pit 2 pounds peaches (no need to peel). Cut into 1/2-inch thick slices. Place the peaches, 1/4 cup packed light brown sugar, 2 tablespoons cornstarch, 1 1/4 teaspoons kosher salt, and 1 teaspoon vanilla extract in a large bowl.

Step 5

Juice half of the zested lemon until you have 1 tablespoon juice, then add to the bowl. Stir to combine.

Step 6

Transfer to the baking dish and spread into an even layer. Break the crisp topping into large clumps with your hands and scatter over the filling in an even layer.

Step 7

Bake until the crisp topping is golden-brown and the filling is bubbling, 35 to 40 minutes. Cover the crisp with aluminum foil if the topping begins to brown too quickly. Let cool for at least 15 minutes before serving with vanilla ice cream if desired.