Mix yeast with a pinch castor sugar and some water in a bowl. Heat in a microwave for 10 seconds. Remove from heat, whisk and set aside to activate.
Sieve refined flour and multigrain flour one after the other into a bowl and mix well. Add salt and 1 tablespoon castor sugar and mix well. Make a well in the center, add activated yeast and mix well. Add some water and knead into soft dough.
Dust the worktop with some refined flour, place the dough and knead till it can be stretched without breaking. Add butter and knead again till well incorporated.
Dust the worktop with some refined flour, divide the dough into 2 portions and roll them into a thin log.
Grease a baking tray with some butter and dust some refined flour.
Place one log horizontally on the worktop and place the other log vertically on it in the centre. Fold the edges of the two logs overlapping each other, till both logs are fully used and seal the open ends.
Place the loaf on the greased baking tray, cover with damp muslin cloth and prove for 20-25 minutes.
Preheat oven at 180º C. Break egg into a bowl, whisk and brush it on top of proven loaf. Place the baking tray in preheated oven and bake for 15-20 minutes. Remove from oven and brush some melted butter on top.
Cool and serve.