1-Thaw and prep the rhubarb: Thaw frozen rhubarb completely in a colander, pat dry, then toss with 1/4 cup granulated sugar, 2 tablespoons lemon juice, and 1/2 teaspoon cinnamon.

2-Cook the rhubarb gently: Transfer mixture to a 9-inch baking dish and cook over medium heat for about 5 minutes, stirring once, until slightly softened and juices release.

3-Make the crumble topping: Combine 3/4 cup rolled oats, 1/2 cup all-purpose flour, 1/2 cup packed brown sugar, and 1/4 teaspoon salt; rub in 1/2 cup cold cubed butter until mixture resembles coarse crumbs with pea-sized butter pieces.

4-Assemble and bake: Spread topping evenly over the cooked rhubarb and bake at 375°F for 30 minutes, until golden brown and filling bubbles around edges.

5-Rest before serving: Let the crisp sit for 5 to 10 minutes so juices settle, then serve warm.