1-Preheat your oven: Begin by preheating your oven to 375°F, which sets the stage for even baking and a perfectly crisp topping. This temperature helps the rhubarb soften while the oats turn golden, creating that irresistible contrast.

2-Prepare the baking pan and rhubarb mixture: First, butter an 8- or 9-inch square baking pan to prevent sticking and add a subtle richness. In a bowl, whisk together the granulated sugar and cornstarch until smooth, which will thicken the juices from the rhubarb. Then, add the rhubarb and lemon juice, tossing gently to coat every piece, before transferring the mixture to the prepared dish.

3-Make the topping and bake: For the topping, combine the oats, flour, brown sugar, salt, and cinnamon in a separate bowl. Stir in the melted butter until the mixture is evenly moistened, leaving some larger clumps for extra crunch. Spread this over the rhubarb layer and bake for 35 to 45 minutes, watching for bubbling juices and a golden brown top.

4-Cool before serving: Once done, let it cool on a wire rack for about 15 minutes to help the flavors settle and make serving easier.