Wash the ＊ rhubarb. Cut off the leaves. We will only be using the stems.
Cut into 1-2 cm wide pieces. Place into a bowl and mix thoroughly with the ＊ sugar. Let it sit.
[Tart Crust] Soften the butter (I used the microwave), and combine with all of the ingredients. Form it into a ball and cover with plastic wrap. Let it sit in the refrigerator.
[Streusel] While the crust is chilling, make the streusel. Without softening the butter, combine all of the ingredients
Use a fork to combine well until it becomes crumbly. Use your fingers at the end.
Grease the gratin dish with melted butter.
Remove the tart crust from the refrigerator and spread it into the dish.
Place the sugared rhubarb from Step 2 into the crust. (Remove the moisture as much as possible.) Top with the streusel.
Bake in a 200°C oven for 40 minutes. After baking, the surface should look like this photo. The rhubarb releases some moisture, but this makes the tart moist and delicious if you let it cool first before serving.
Story Behind this Recipe
Rhubarb is in season during the spring. I learned this recipe from a neighbor. Apparently you're supposed to use a tart pan, but I didn't have one so I used a gratin dish (10 x 20 x 3 cm). I halved the recipe to accommodate the dish.
Many cakes in Germany are topped with a sugary, buttery, crumbly topping. Rhubarb is pretty sour by itself which is why I mixed it with sugar. The recipe may seem quite sweet, but when eaten, the sweet and tart taste is delicious.