Preheat the oven to 375°F. If using fresh peaches, blanch 30–45 seconds in boiling water and plunge into ice water, then peel and slice into 1/2-inch wedges. For canned peaches, drain and pat dry.

In a large bowl toss peaches with granulated sugar, lemon juice, lemon zest, ground cinnamon, cornstarch (or instant tapioca), and salt. Let sit 10 minutes to hydrate the starch and meld flavors.

In a medium bowl combine flour, rolled oats, brown sugar, cinnamon, and salt. Cut in cold butter with a pastry cutter or two forks until mixture resembles coarse crumbs with some pea-sized pieces; stir in nuts if using.

Spread the topping on a parchment-lined sheet pan and bake 8–12 minutes at 375°F, until lightly golden at the edges. Remove and cool while assembling the crumble.

Spoon the peach filling into a 9x13-inch baking dish and spread evenly. Scatter the par-baked crumble topping over the peaches, pressing lightly so crumbs contact the fruit. Place the baking dish on a sheet pan and bake at 375°F for 25–30 minutes until juices bubble at the edges and topping is deep golden. Tent with foil if topping browns too quickly.

Remove from the oven and let rest 10–15 minutes so the filling sets. Serve warm with vanilla ice cream or whipped cream.