Preheat your oven to 350°F (175°C). Place an oven rack in the center position so the topping browns evenly.

Take out a 2-quart oven-safe glass casserole dish (about 8x8 inches or similar). Do not grease it; the butter in the topping will provide enough richness.

Dump the rock-hard frozen raw peach slices directly into the casserole dish and spread them into an even layer. It’s fine if they’re clumped or frosty; do not thaw first.

In a medium mixing bowl, whisk together the granulated sugar, all-purpose flour, and ground cinnamon until no streaks of flour remain and the cinnamon looks evenly distributed.

Pour the melted butter over the sugar-flour-cinnamon mixture. Stir with a spoon or spatula until you have a thick, slightly crumbly batter. It should be moist and clumpy, not pourable like cake batter.

Using your fingers or a spoon, scatter the buttery mixture evenly over the frozen peaches, covering as much of the surface as you can. It’s fine if a few peach pieces peek through; they’ll bubble up as they bake.

Place the casserole dish on a rimmed baking sheet to catch any bubbling juices. Transfer to the preheated oven.

Bake for 40–50 minutes, until the top is golden brown, the edges look crisp, and you can see peach juices bubbling up around the sides and through a few spots in the topping.

Remove from the oven and let the dessert rest on a cooling rack for at least 15–20 minutes. This pause allows the hot juices to thicken slightly so you get saucy peaches instead of a runny pool.

Spoon warm servings into bowls and, if you like, top with ice cream, whipped cream, or yogurt. Store any leftovers covered in the refrigerator for up to 3 days, and rewarm gently in the oven or microwave before serving.