Preheat oven to 350ºF. Whisk flour and sugar together in a medium bowl. Using a pastry blender (or your fingers), work the butter into the flour until the mixture resembles coarse crumbs. Dump the crumbs into a parchment-lined 9x13" baking pan and use your palms to press it evenly into the bottom (it's super crumbly, but will come together as soon as you start pressing it in). Bake for about 20 minutes, until golden.
While the crust bakes, combine rhubarb, sugar and water in a medium saucepan over medium-high heat. Bring to a boil, then reduce heat to medium and cook, stirring frequently, until rhubarb breaks down to a sauce - about 15 minutes. Stir in lemon juice. Remove from heat and cool 5 minutes.
Meanwhile, whisk eggs and yolks together in a medium bowl. Very slowly, about a tablespoonful at a time, whisk 1 cup of the rhubarb sauce into the eggs, then gradually whisk the egg-rhubarb mixture back into the saucepan with the remaining rhubarb.
Scrape curd on top of warm crust, spreading evenly to edges. Bake another 30 minutes, until set and slightly puffed (it will still wiggle a bit - it'll firm up as it cools). Cool in pan placed on wire rack. Once at room temperature, dust with confectioners' sugar using a fine-mesh sieve. Store leftovers in the fridge.