Peel the potatoes and cut into 3cm chunks. Finely chop the onion and carrot. Crush or finely chop the garlic. Finely chop the thyme leaves, discarding the tough stems. Crumble 1 stock cube* into a heatproof jug, add 250ml (1 cup) boiling water and stir to dissolve.
Step 2 — Make mash
Put the potato in a medium saucepan of cold salted water and bring to the boil. Reduce the heat to medium and cook for 12 mins or until tender. Drain well and return to the pan. Mash the potato until smooth. Add 20g butter, season with salt and pepper and stir until combined.
Step 3 — Start filling
While the potato is cooking, heat 1 tbs olive oil in a separate medium saucepan over medium-high heat. Add the onion, carrot, garlic, thyme, pepper and garlic seasoning and a pinch of salt and cook, stirring occasionally, for 4-5 mins until softened.
Step 4 — Prep salmon
Meanwhile, preheat the grill to high. Put the salmon on a board and using a sharp knife, carefully slice between the skin and flesh to remove the skin. Discard the skin and cut the salmon into 3cm chunks.
Step 5 — Finish filling
Add 2 tsp olive oil and 2 tbs flour to the vegetables and cook, stirring, for 1-2 mins until foaming. Gradually add the stock and 125ml (½ cup) milk, stirring, to prevent lumps forming. Add the peas, then bring to a simmer and cook, stirring, for 2 mins. Add the salmon and 2 tsp dijon mustard and gently stir to combine. Remove from the heat.
Step 6 — Grill potato crust
Transfer the salmon filling to a 1.5L (6 cup) baking dish. Spoon the mash over the top. Using a fork, evenly spread the mash to cover the filling and create ridges. Spray with olive oil (or brush with 2 tsp olive oil) and season with salt and pepper. Grill the pie for 8-10 mins until golden and the salmon is just cooked through. Take the pie to the table and serve.