Preheat the oven to 300°F. Heat the oil in a flameproof casserole or Dutch oven until it is very hot. Brown the meat in batches, taking care not to crowd the pan, for 3 - 4 minutes each side, then remove from the pan and set aside.

Add the onions to the casserole with the garlic and cook for 2 minutes. Sprinkle the flour on top and stir in with a wooden spoon, scraping the base of the pan. Cook for about 1 minute, stirring constantly, until the mixture is golden brown. Gradually stir in 3/4 cup water and tomato paste, and season with salt and freshly ground black pepper. Continue stirring until the mixture begins to thicken, then return the meat to the pan, add the mushrooms and bring to a boil. Cover and either cook gently on the stove or bake in the oven for 1-1/2 hours.

Begin preparing the biscuits no more than about 10 minutes before the beef has finished cooking. Sift the flour and salt into a wide bowl, then add the butter and rub it in with a flicking action of the thumb across the tips of the fingers. When the mixture resembles fine bread crumbs, add the herbs. Stir in the buttermilk, using a round-bladed knife, until the flour has disappeared and the mixture is in large lumps, then bring together quickly into a rough ball. Place on a lightly floured surface and knead quickly until just smooth. Roll or pat out the dough with the palm of your hand to a 5/8-inch thickness, then cut out about ten circles using a 2-inch cutter.

Remove the stew from the oven, then increase the temperature to 400F. Arrange the biscuits on the surface of the stew and brush the tops with the egg. Place the stew at the top of the oven, uncovered, to cook for 12 minutes, or until the biscuits have risen and turned golden brown.