Activate the yeast: in the lukewarm milk, stir in the sugar and yeast, then let it stand for 10 minutes until foamy.
In a large bowl, whisk together the flour and salt, then rub in the butter with your fingertips until the mixture resembles coarse crumbs.
Add half of the grated cheese and the mustard, then mix with the flour mixture.
Add the sour cream, egg, and activated yeast, then knead the dough thoroughly until smooth.
On a lightly floured surface, roll out the dough to about 2 cm (¾ inch) thickness, then fold it in thirds to create layers.
Cover with a clean kitchen towel and let rest for 30 minutes to allow the dough to rise slightly.
Roll out the dough again to about 2 cm (¾ inch) thickness, then use a scone cutter to cut out the shapes.
Place the scones on a baking sheet lined with parchment paper, and brush the tops with beaten egg.
Sprinkle the tops with the remaining grated cheese, then bake in a preheated oven at 350°F (180°C) for 20-25 minutes, or until golden brown.
Let cool slightly, then serve fresh, with a crispy exterior and a soft, crumbly interior.
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