In a large mixing bowl, combine the sourdough starter, warm water, sugar, and salt. Mix until combined, then gradually add the flour, stirring until a soft dough forms.

Turn the dough onto a floured surface and knead for about 5 minutes until it becomes smooth and elastic.

Cover the dough with a damp cloth and let it rise in a warm place for about 30 minutes, or until doubled in size.

Preheat the oven to 425°F (220°C).

Punch down the risen dough and divide into 24 equal pieces. Roll each piece into a small ball.

Bring a pot of water to a boil and add the baking soda.

Drop the dough balls into boiling water, cooking for about 30 seconds each to form the classic pretzel crust.

Using a slotted spoon, remove the pretzels and place them on a parchment-lined baking sheet.

Brush each pretzel with the beaten egg and sprinkle with coarse sea salt.

Bake in the preheated oven for 12 to 15 minutes, or until golden brown.