To make the roux, add the flour and water to a saucepan. Heat over a moderate heat and whisk continuously until the mixture thickens to a slurry, peanut butter like texture. Remove from the heat and continue to whisk for a further minute. Pour into a bowl and set aside to cool for 5 minutes. Then place in the fridge to speed up the cooling process.
To a large mixing bowl, add the flour, sugar, salt, baking powder and yeast. Mix and then make a well in the centre and pour in the sunflower oil, oat milk, melted margarine and the lukewarm roux. Stir the dough mixture until well combined. Then turn out onto a well floured surface.
Knead the dough for 15 minutes until smooth and elastic. This is a wet sticky dough so flour the surface and your hands as necessary.
Place the dough in a clean well oiled mixing bowl. Then turn over to coat the whole surface of the dough in oil. Cover with a damp tea towel and leave to rise for 1 hour.
When the dough has roughly doubled in size, knock the air out of the dough. Turn out onto the work surface and lightly knead for 30 seconds. Split the dough into 12 pieces and roll into balls.
Place the dough balls 2cm apart in a deep baking tin, cover with a damp tea towel and leave to rise for 45 minutes.
Place the tray in a preheated oven at 220°C / 200°C fan / gas 7 for 15 minutes. The buns are ready when well risen and golden-brown on top.
Make the glaze by mixing the boiling water and sugar.
Remove the buns from the oven and immediately brush with the glaze. Eat warm or leave to cool completely.