Preheat the oven to 180°C.
Prepare the biscuit dough, by mixing finely chopped butter and all other biscuits ingredients until combined. Do not over work the dough. Place your dough in the fridge for 30 minutes.
Finely chop the potato, carrots, celeriac, and boil them in a pot of salted water until tender. Make sure that your veggies are not overcooked. Drain and set them aside.
Chop the courgette and separate broccoli florets. Set them aside.
To prepare the sauce, melt butter in a large frying pan, add finely chopped onion and cook over low heat until soft. Add 3 tbsp flour and mix well, pour the chicken broth and milk and mix again until combined.Make sure that the sauce is smooth, without any lumps. Add the courgette, broccoli florets, thyme, salt, and black pepper. Cook over low heat for a few minutes, until courgette and broccoli are tender and sauce slightly thicken.
In a large ovenproof dish mix sauce, boiled veggies, and cooked chicken. Set aside.
To make the biscuits, transfer the dough to a floured surface and roll out slightly with a rolling pin. Cut out your biscuits using a cookie cutter or glass. Place your biscuits on a baking sheet lined with parchment paper, and pre-bake them for 10 minutes.
Transfer your pre-baked biscuits on top of your pie, brush them with egg wash, and bake all together for another 10-15 minutes. Serve hot.