1.Toss chicken in flour to coat, shake off excess. Reserve excess flour. Heat oil in a large frying pan, cook chicken, in batches, until browned. Remove from pan. Wipe pan with absorbent paper.
2.Heat butter in same pan, cook onion, carrot and celery, stirring, until softened. Add garlic, cook, stirring, until fragrant. Stir in reserved excess flour, then stock and mustard, stir over high heat until mixture boils and thickens.
3.Place celeriac in a 4.5-litre (18-cup) slow cooker, top with chicken then onion mixture. Cook, covered, on high, 3 hours.
4.Meanwhile, place broad beans in a medium heatproof bowl, cover with boiling water, stand 2 minutes, drain. Peel away grey skins.
5.Add broad beans to cooker, cook, covered, on high, 30 minutes. Season to taste.
6.Roast the walnuts, in a preheated 180°C (160°C fan forced) oven, for 5 minutes or until browned lightly.
7.Serve sprinkled with nuts and celery leaves.
To freeze Complete the recipe to the end of step 3. Pack into freezer-proof containers, leaving a 2.5cm (1 inch) space to allow for expansion. Seal, label and freeze for up to 3 months. Thaw overnight in the fridge. Reheat in a saucepan or microwave on medium power (50%) until heated through, then continue from step 4.