Icelandic Happy Marriage Cake (Hjónabandssaela)

Make the filling: Combine rhubarb, water, sugar and salt together in a large saucepan. Heat over medium until it begins to bubble and then turn down to low. Cover and simmer for 5 minutes. Remove lid and continue to simmer until the rhubarb has broken down and the mixture has thickened, about 10 minutes more. Stir in vanilla extract. Set aside to cool to room temperature.

Make the crust and topping: Place the oats, flour, brown sugar, baking soda and salt in the work bowl of a food processor. Pulse a few times to combine. Cut the butter into tablespoon-sized pieces and scatter over the top of the dry ingredients. Pulse until the butter is incorporated. With the machine running, add the egg and vanilla extract through the feed tube and process until a cohesive dough has formed.

Press about ⅔ of the dough evenly into the bottom and up the sides of a 9 inch tart pan with a removable bottom. Cover the crust and wrap the remaining dough tightly with plastic wrap. Chill for 20-30 minutes.

Meanwhile, preheat the oven to 350 degrees F. Spread the rhubarb filling in an even layer over the crust. Crumble pieces of the remaining dough over the top of the tart. Bake until golden brown and bubbly, about 30-35 minutes. Serve warm or at room temperature.