Boil potatoes in salted water for 15-20 mins until tender. Drain and steam dry.
Brown chicken in garlic-infused oil for 3-4 mins, then set aside. Fry mushrooms for 5-7 mins until golden and moisture evaporates. Add carrot, leek greens and thyme; cook for 5–7 mins.
Stir flour into vegetables for 1 min. Gradually add stock, then milk. Add Dijon, Worcestershire, seasoning and MSG. Simmer 4-6 mins until thick, then stir in soft cheese. Return chicken and cook gently until cooked through and sauce coats everything.
Mash potatoes with butter, warm milk and 20g cheddar. Season. Spoon filling into a baking dish, cover with mash and make fork ridges. Sprinkle over remaining 25g cheddar and Parmesan.
Bake at 200°C fan for 20-25 mins until golden and bubbling. Grill 2–3 mins if needed. Rest 10-15 mins, then garnish with chives.
