Preheat oven to 220˚C. Line a large baking tray with baking paper and set aside.
Heat a drizzle of olive oil in a large frying pan. Fry the mushrooms in batches until golden brown and they have released all their moisture. Season. Remove mushrooms from heat and spread out in a single layer onto some paper towel. Cover with another layer of paper towel and gently squeeze any excess moisture out.
Place flour, baking powder, salt, pepper and mustard powder into a large mixing bowl and whisk together until well combined.Rub the butter into the dry mixture using your fingertips until it resembles wet sand. Add in the cooked mushrooms and toss well so that they are evenly coated with flour.
Add in most of the cheese, reserving a handful to top the scones. Add in the chives, and stir to combine. Pour in the milk and water and mix until the dough just comes away from the edge of the bowl. Tip on to a very lightly floured surface and bring the dough together into a rectangle pressing it gently together and never kneading or overmixing it. Cut out scones with a fluted cutter. Press excess together gently, avoiding overmixing.
Place scones on a baking tray and brush with the beaten egg mixture.Top with the remaining cheese and bake for about 20 minutes until evenly golden brown and cooked through. Rotate the baking tray half way through the bake for a more even colour and even rise. Allow to cool on a wire rack before breaking open and and smearing with a generous amount of butter.