In a large bowl, whisk together the flour, sugar, salt and yeast. Whisk the egg yolk, milk and lemon zest into the melted and cooled butter until combined. Drizzle the wet mixture onto the dry mixture. Use your hands to mix the ingredients together until you have a shaggy looking dough.

Turn the dough out onto a lightly floured surface and knead it for a few minutes until smooth. Form into a ball, place it back in the bowl, cover with a clean tea towel and place it somewhere warm away from drafts to proof until doubled. This will take approximately 1.5 hours at room temperature (21℃, 70℉).

Line a 9 x 13" baking pan with parchment paper, allowing the sides of the parchment to hang over the edge like a sling. Set aside. In a medium bowl, mix the streusel ingredients together with your fingers until it resembles course crumbs. Place in the fridge to chill.

Once the dough has doubled in size, place it on a lightly floured surface and press it down with your hands into a rectangular shape. Lightly dust the top of the dough with flour, then roll it out to the size of the baking pan. Lightly dust the top with flour again, fold the dough into quarters, place it in your baking pan and unfold it, pressing the dough to the edges and corners with your fingers if needed. Make dimples all over the dough with your fingers, then cover and set it aside until the dough has become slightly puffy. This will take approximately 30-40 minutes at room temperature (21℃, 70℉).

While the dough is proofing, preheat the oven to 350℉. Mix the chopped rhubarb with sugar in a medium bowl and set aside.

Once the dough has risen, evenly distribute the rhubarb over it, gently pressing the rhubarb into the dough. Then sprinkle the streusel over the rhubarb. Bake for 50 minutes or until any visible cake is golden brown. Remove and allow to cool in the baking pan for 30 minutes before using the parchment sling to transfer it to a cutting board. Cut the cake into squares or any shape you like before serving. Best served with a big dollop of slightly sweetened whipped cream.