Preheat the oven to 350°F (180°C).
In a medium bowl, combine 1 cup all-purpose flour, a pinch of salt, ¼ cup white sugar, and ⅓ cup light brown sugar.
Drizzle ½ cup unsalted butter, melted, over the dry ingredients and stir with a fork until it resembles coarse, clumpy crumbs. Cover and keep refrigerated while you make the filling or up to a week. Or frozen for a month.
Have ready an oven-proof glass or ceramic, deep baking dish.
Scatter 4 to 5 cups rhubarb pieces in the dish and sprinkle with 2 tablespoons light brown sugar and drizzle with 2 teaspoons fresh lemon juice. Grate ½ teaspoon lemon zest over them and stir to mix lightly.
If using 2 teaspoons cornstarch or flour to thicken the juices, dissolve them first in the lemon juice before drizzling it over the sugar.
Top the rhubarb with this mixture.
Bake for about 40 minutes, until dry, golden, and the reddish fruit juices are bubbling.
Let cool on a wire rack until warm and eat with some whipped cream or vanilla ice cream.
Keep leftovers refrigerated, covered in plastic wrap or in an airtight container, and warm slightly before eating to bring back the crispiness of the topping.
