How To Warm Up This Winter With Cheese Knishes!

Name: How To Warm Up This Winter With Cheese Knishes!
Cuisine: European, British
Ingredients: 2 cups bread flour; 2 teaspoons fine salt; 113g/1 stick cold butter, cut into cubes; 225g/8ozs. sour cream (full fat); 500g/1.1lbs. potatoes; 1 tablespoon butter; 3 onions, peeled and finely chopped; 2 cloves garlic, peeled and finely chopped; 2 cups cheese (I used a mix of ricotta, cream cheese and parmesan); 3 tablespoons chopped parsley; salt and pepper for seasoning; 1 egg for egg wash; Hot mustard to serve