In a bowl, mix the cold butter cut into cubes with flour until you get a sandy consistency. Add the water, continue to knead, add 30 g of sugar and salt. If you still need water, add 1-2 more tablespoons.
Refrigerate the dough wrapped in cling film and leave for at least 30 minutes.
Mix the cherries with starch and the rest of sugar until all the fruits are covered.
After 30 minutes, spread the dough in a thin sheet on a surface lined with flour and place it in a tray lined with baking paper. Add the fruit and place the edges of the dough over the fruit compositiom but let the middle empty and grease the tart with the beaten egg.
Cook the galette in the preheated oven at 180 ° C for 30-40 minutes. After the tart is ready, leave it to cool before being cut. You can serve it simple or with some ice cream.