Step 1Preheat oven to 160°C. Boil the potatoes and cauliflower in a saucepan for 10–12 minutes, or until tender. Drain well. Return to pan and place over a low heat; mash vegies until smooth. Stir in 1 tablespoon of sour cream, 1 tablespoon of chives and all the baby spinach. Cook, stirring, until spinach just wilts.
Step 2Transfer the mash to a 4-cup capacity ovenproof baking dish. Sprinkle with parmesan. Bake for 15 minutes, or until top is golden.
Step 3Meanwhile, spray a large non-stick frying pan with oil and place over a medium-high heat. Cook chicken for 3–4 minutes each side, or until golden brown and cooked through. Transfer chicken to a plate, and cover to keep warm.
Step 4Return the frying pan to a medium heat; add the garlic and wholegrain mustard, and stir to combine. Add the stock, bring to the boil, then simmer for 2 minutes, or until reduced by half. Remove pan from heat, stir in the remaining sour cream and chives. Season the sauce with cracked black pepper.
Step 5Serve chicken with the gratin and steamed green beans, drizzled with the mustard sauce.