How to Make It
In a large saucepan, bring the water to a boil. Add the sugar and stir to dissolve it. Add the rhubarb and cook over low heat until softened but not falling apart, about 5 minutes. Using a slotted spoon, transfer 1/2 cup of the rhubarb to a small bowl and reserve for the filling.
Preheat the oven to 400° and line a large baking sheet with parchment paper. In a food processor, combine the farmer cheese with the 4 tablespoons of softened butter, the confectioners' sugar, cake flour, egg yolks, lemon zest and vanilla seeds and puree. Add the sour cream and process until creamy. Scrape the mixture into a large bowl.
In another bowl, using a handheld electric mixer, beat the egg whites at high speed until soft peaks form. Gradually add the granulated sugar and beat until the whites are stiff and glossy. Fold the whites into the cheese filling.
Brush 5 sheets of phyllo with some of the melted butter, stacking them. With a long side of the phyllo facing you, spread half of the cheese filling along the bottom edge. Spoon 1/4 cup of the reserved rhubarb over the cheese filling. Carefully roll up the strudel, tucking in the sides as you go. Using spatulas, transfer the strudel to the prepared baking sheet. Repeat with the remaining phyllo, cheese filling and 1/4 cup of reserved rhubarb, and some of the melted butter. Transfer to the baking sheet, leaving a few inches between the strudels.
Brush the strudels with the remaining melted butter and bake them in the center of the oven for 45 minutes, turning the baking sheet once halfway through, until deeply golden (some of the filling may leak out). Let the strudels cool for 30 minutes.
In a medium saucepan, combine the milk with the sugar and vanilla bean and bring to a simmer. In a small bowl, whisk the egg yolks with the cornstarch. In a thin stream, whisk the egg yolks into the hot milk and simmer until the sauce is thickened and bubbling, about 3 minutes. Strain the sauce into a heatproof bowl. Cover with plastic wrap and refrigerate.
Slide the strudels onto a work surface and slice each one into 4 or 5 pieces. Serve the strudel with the crème anglaise and the remaining rhubarb compote.