Preheat your oven to 375°F (190°C). Grab an 8x8-inch or similar oven-safe glass baking dish. There’s no need to grease it.
Pour the frozen raw peach slices straight from the bag into the glass baking dish. It’s fine (and ideal) if they’re frosty and stuck together in spots—no thawing needed. Spread them into an even layer as best you can.
In a medium bowl, whisk together the granulated sugar, all-purpose flour, and ground cinnamon until everything looks evenly mixed and there are no floury clumps.
Drizzle the melted butter over the dry mixture and stir with a fork until you get a crumbly, sandy mixture. It should look a bit like damp brown sugar with some small clumps—that’s your simple topping.
Sprinkle this buttery mixture evenly over the frozen peaches in the baking dish, covering as much of the surface as you can.
Place the baking dish on the middle rack of the preheated oven. Bake for 35–45 minutes, or until the peaches are tender and bubbling around the edges and the topping is golden brown.
Carefully remove the dish from the oven and let the dessert rest for at least 10–15 minutes.
Spoon the warm peach dessert into bowls and top with ice cream or whipped cream, if you like.
