Preheat your oven to 350°F (175°C). Line your baking dish with parchment paper or grease it lightly.
Mix condensed milk and flour in a bowl until smooth. It will be a thick, sticky batter.
Fold in the diced rhubarb. Don’t overmix—just stir until it’s evenly distributed.
Spread the mixture into the baking dish. Use a spatula to smooth out the top.
Bake for 35 to 40 minutes, or until the top is golden and a toothpick comes out mostly clean.
Let it cool completely before slicing. Dust with confectioner’s sugar if you like.