Preheat the oven to 200C, fan-forced. Put an oven tray on the lower oven shelf to heat. Finely chop the carrot and onion. Cut the fish into 3cm chunks and put into a bowl. Add 3 tsp pepper and garlic seasoning (reduce the seasoning for a milder pepper kick) to the fish and toss to coat. Bring a small saucepan of water to the boil for the peas.
Step 2 — Make sauce
Melt 20g butter in a medium deep frypan over medium-high heat. Cook carrot and onion, stirring regularly, for 5 mins or until softened. Add 1 tbs flour and cook, stirring, for 1 min. Gradually stir in 125ml (½ cup) milk and 125ml (½ cup) hot water, stirring constantly to prevent lumps forming. Reduce heat to medium and cook for 6-8 mins until thickened. Remove from heat.
Step 3 — Cook filling
While the sauce is cooking, heat 1 tbs olive oil in a small frypan over medium-high heat. Cook the fish, turning, for 2-3 mins until golden. Remove from the pan. Cook the peas in the pan of boiling water for 2 mins. Drain. Gently stir the fish and peas into the sauce until combined. Taste, then season with salt.
Step 4 — Prep filo pie base
Spray a 1L (4 cup) shallow baking dish with olive oil (or brush with olive oil). Put 1 filo sheet on a bench (keep remainder covered with a damp tea towel to prevent drying). Spray with olive oil. Top with another filo sheet at an angle and spray with olive oil. Repeat with a further 6 filo sheets. Transfer to the baking dish. Trim the edges with scissors, leaving 3cm overhanging.
Step 5 — Assemble pie and bake
Spoon the filling into the filo pie base. Fold in the filo edges to partially cover the filling. To complete the pie top, spray 2 filo sheets with olive oil, loosely scrunch each filo sheet and place over the filling to cover (the remaining filo won't be used in this recipe). Spray the pie top with olive oil and put on the heated tray. Bake for 20-25 mins until golden.
Step 6 — Dress spinach and serve
Just before serving, put 1 tbs extra virgin olive oil and 2 tsp white wine vinegar in a large bowl, season with salt and pepper and whisk to combine. Add the spinach to the dressing and toss to coat. Serve the pie at the table with the spinach salad.