Mix the flour, salt, sugar and yeast in a bowl. Add the butter, milk and egg white, mix and knead to form a soft, smooth dough. Cover and leave to rise at room temperature for approx. 1 hr. until doubled in size.
Shape the dough into a roll, cut into 12 pieces and shape into balls. Make a hole in the middle of each ball with your finger. Allow the dough rings to form around your finger until the opening measures approx. 3 cm in diameter. Place the dough rings on a baking tray lined with baking paper, cover and leave to rise for approx. 15 mins.
Place 3 bagels at a time in just-boiling, salted water for approx. 15 secs. Remove with a slotted spoon, drain thoroughly and return to the baking paper. Brush the bagels with egg yolk, sprinkle with sesame seeds and salt.
Bake for approx. 15 mins. in the centre of an oven preheated to 220°C. Remove from the oven, allow to cool on a cooling rack.
Cut the bagels in half crosswise, spread with Cantadou. Fill the bagels with salmon, radish and cress.