Method:
1 Preheat oven 200°C fan-forced. 
2 Heat oil and butter in a saucepan over medium heat, add onion, garlic, leek and cook for 3-4 minutes or until onion soften and caramelize a little. Stir in flour and cook for 1 minute.  Slowly stir in chicken stock and cook for 1-2 minutes or until sauce starts to thicken.  Add perch, season with salt and pepper and cook for 2-3 minutes.  Transfer mixture to an ovenproof pie dish.
3 Place potatoes in a microwave-safe bowl, cover and microwave on High/100% for 5 minutes or until tender.  Using a potato ricer, mash the potatoes until smooth. Add butter and season with salt and pepper and stir to combine.  Transfer mixure to a piping bag and pipe the mix evenly on the top of the pie, scatter with thyme leaves. 
4 Bake for 5-6 minutes or until top is golden.  Serve.