Combine the trimmed and cut rhubarb, water, sugar, and salt in a medium saucepan over medium-high heat. Bring to a boil, then cover and simmer for 5 minutes to soften the rhubarb. Uncover the pot and continue simmering for another 10 minutes, stirring occasionally, until the mixture thickens slightly and the rhubarb breaks down into a jammy consistency. Remove from heat, stir in the vanilla, and set aside to cool completely. I prefer to let this cool to room temperature before assembling so it doesn't melt the dough base.

Preheat your oven to 350°F. In a large bowl, combine the oats, flour, brown sugar, baking soda, and salt, mixing until evenly distributed. Add the cold butter cubes and work them into the dry mixture using your fingertips or a pastry cutter until the mixture resembles coarse breadcrumbs with some pea-sized butter pieces still visible. In a small bowl, whisk together the egg and vanilla, then add to the oat mixture and stir until a shaggy dough forms. I find that working quickly with cold butter keeps the texture more crumbly and creates a better final result.

Press approximately two-thirds of the dough firmly into the bottom of a 9x13 inch baking dish, creating an even base layer. Cover the dish with plastic wrap and refrigerate both the base layer and the remaining dough for 20-30 minutes until firm. This chilling step prevents the dough from spreading too much during baking and helps maintain the crumbly texture.

Remove the chilled dough from the refrigerator and carefully spread the cooled rhubarb filling evenly over the pressed dough base. Crumble the remaining dough pieces over the filling in an even layer, leaving some gaps so the rhubarb shows through slightly. Bake at 350°F for 30-35 minutes until the top is golden brown and the filling bubbles slightly at the edges. The cake is done when the crumble topping has set and turned a light to medium golden color.

Remove the cake from the oven and allow it to cool for at least 15 minutes before cutting. This rests the filling and makes it easier to cut clean pieces. Serve warm or at room temperature—both are delicious. The cake keeps well for 2-3 days covered at room temperature.