Preheat the oven to 160° C w/ fan.
Bowl #1: mix flour, baking powder, sugar & salt. Add cold butter cut into cubes & knead everything together w/ your hands. Add the egg & knead again until you get more or less fine crumbs. Set aside.
Zest the organic lemon.
Bowl #2: Whisk ricotta, sugar & lemon zest until combined.
Clean & trim the rhubarb. Cut the stems into pieces (about 2cm). You need about 200 g.
Add about ⅔ of the crumbs to the tart form & press to form bottom & edge.
Pour the ricotta mess onto the tart form & add the rhubarb.
Add the rest of the crumbs on top.
Bake in the preheated oven for about 40-45 min. It should get a nice light brown colour.
Serve still warm or let it cool sown & store in your fridge.