Peel the sweet potatoes and cut into 4cm chunks. Put in a medium saucepan, cover with cold salted water and bring to the boil. Reduce the heat to medium and cook for 12-14 mins until tender. Drain and return to the pan, then stir over low heat for 10-20 secs until any water is evaporated (drying the sweet potato helps to make fluffy mash). Remove from the heat.
Step 2 — Prep ingredients
Meanwhile, finely chop the onions. Finely chop the carrots. Trim the beans. Finely chop the coriander stems and coarsely chop the leaves. Crush or finely chop the garlic. Peel and finely grate the ginger. Thinly slice the chicken into strips.
Step 3 — Start filling
Heat 1 tbs oil in a large deep frypan over high heat. Cook the chicken, stirring, for 5 mins or until golden and cooked through. Remove from the pan. Heat 2 tsp oil in the pan over medium-high heat. Add the onion and carrot and cook, stirring occasionally, for 7-8 mins until softened.
Step 4 — Simmer filling
Add garlic, ginger and coriander stems to the onion mixture and cook, stirring, for 1 min or until fragrant. Add 1 tbs flour and cook, stirring, for 1-2 mins until golden. Gradually add cream and 125ml (½ cup) water, stirring constantly to prevent lumps forming. Bring to a simmer, reduce heat to medium and cook, stirring, for 2-3 mins until thickened. Preheat grill to high.
Step 5 — Assemble pie
Stir the chicken and any resting juices, curry paste and 1 tbs soy sauce into the filling. Taste, then season with salt and pepper. Transfer the chicken filling to a 1.5L (6 cup) baking dish. Mash the sweet potato until smooth, then stir in 2 tbs milk. Taste, then season with salt and pepper. Spoon mash over filling and use a fork to create ridges. Reserve saucepan.
Step 6 — Grill pie and serve
Grill the pie for 8 mins or until golden. While the pie is grilling, rinse the reserved pan, half-fill with water and bring to the boil. Cook the beans in the pan of boiling water for 3 mins or until just tender. Drain. Season the beans with salt and pepper. Scatter the remaining coriander over the pie. Serve the pie at the table with the beans.