Preheat the oven to 230°C/445°F. Whisk egg yolk with a little bit of water in a wide, rimmed dish. Dip each ring into the egg yolk-water mixture, then transfer to a wide, rimmed dish of sesame seeds. Transfer to a parchment lined-baking sheet and let rise for approx. 15 min. Bake for approx. 15 min.