Heat the oven to 350°F and lightly grease a 9x13 baking dish. The pan should be ready before you assemble the layers so the crumble stays cold and easy to work with.

Place the rhubarb, sugar, corn starch, cold water, and vanilla in a saucepan over medium heat. As the mixture warms, the rhubarb will begin to soften and release juice.

Cook the filling for about 8 minutes, stirring now and then, until it thickens. It should look glossy and spoonable, almost like pie filling or a loose jam rather than a thin fruit mixture.

Take the saucepan off the heat once the filling is thick. The mixture will continue to settle a bit, and it should coat a spoon lightly.

Combine the oatmeal, flour, brown sugar, and butter with a pastry cutter or fork until a crumble forms. You should see both fine crumbs and some pea-sized buttery pieces, which is exactly right.

Press 3/4 of the crumble mixture firmly into the bottom of the prepared baking dish. An even, compact layer helps create a solid base that won’t fall apart after baking.

Spread the warm rhubarb filling evenly over the crust. Try to cover the surface edge to edge so every bar gets a balanced layer of fruit.

Warm the strawberry jam if using, drizzle it over the filling, and swirl it gently with a knife. The jam adds a little extra fruit sweetness and a prettier pink tone through the center.

Scatter the remaining crumble over the top. Leave it loose and craggy so the topping bakes with crisp little ridges.

Bake for 30 to 35 minutes, then cool completely before slicing. The top should be golden brown and the filling bubbly at the edges, and the center may still seem slightly soft but will firm up as it cools.