Mix the yeast with 300ml lukewarm water. Put the flour, sugar and 1 tsp salt in a large bowl and mix together. Pour over the yeasty liquid and mix into a rough dough.
Tip out onto the work surface and knead together until smooth and elastic - this should take around 10 mins.
Put the dough in a lightly oiled bowl and cover with a piece of oiled cling film. Place in a warm area and leave until doubled in size, about 1 hr, then uncover and tip onto your work surface.
Divide the dough into 10 portions and form into balls - I like to weigh them to make sure they're all the same size. Line up on 2 parchment-lined baking trays and cover lightly with cling film.
Leave for around 30 mins or until risen and puffy, then remove the cling film.
Use a floured finger to make a hole in the centre of each bagel, swirling it around to stretch the dough a little, but being careful not to knock out too much air. Heat oven to 180C/160C fan/gas 4.
Fill a large saucepan with water and bring to the boil. Add the bicarbonate of soda to alkalise the water. Place 1-2 of the bagels in the water at a time and boil for 1 min (2 mins if you want a chewier bagel), turning over halfway through. Using a slotted spoon, lift out the bagels, drain well and place back onto the baking tray.
Brush the bagels with the egg white and sprinkle with your chosen seeds. Bake for 20-25 minus or until golden brown. Transfer to a wire rack to cool before eating. They will keep for 3-4 days, or freeze for 2 months.