Prepare all vegetables and have the rest of the ingredients ready.
Slice potatoes so that they are 0.5cm thick. Place in a saucepan, cover with water, add a pinch of salt and bring to boil. When it starts to boil, lower the heat to medium-high and cook for 3-5 minutes or until the potatoes are almost soft. Drain and set the potatoes on a plate aside.
Add 2 tablespoon olive oil into a large frying pan (use an oven-proof skillet if available). Set on medium-high heat. Add chopped carrots and fry for 5 minutes. Stir occassionally.
Boil a kettle of water. Preheat the oven to 200C/180C fan.
Add courgettes, celery sticks, onion and garlic to the pan with carrots. Season with rosemary and thyme and a pinch of salt and pepper. Toss everything and leave to cook for 5 minutes, stirring every now and then.
Poach the fish
In the meantime, poach the frozen fish - skip this step if using defrosted/chilled fish. Fill ¾ of a saucepan (same one as for boiling the potatoes) with boiling water from the kettle. Set on high heat and bring to boil. Add the frozen fish fillets/chunks. Keep the heat on high so that it starts to boil soon after. Once it boils again, lower the heat to medium-high and leave to simmer for 2 minutes.
Drain the fish fillets (using a sieve) and transfer them carefully to the pan with vegetables. using a wooden spoon, break up the fish fillets into smaller chunks and stir with the cooked veg. Leave to cook on medium heat whilst preparing the white sauce.
Make the white sauce
Add butter into a non-stick saucepan and set on medium heat. Once the butter has melted, add flour and stir until you get a thick, batter-like texture. Fry for a minute and gradually add milk. Use a whisk to stir (whisk) the sauce. The sauce will get thicker. Keep adding the remaining milk. Keep whisking so that it doesn't burn from the bottom.
When the white sauce reaches cream-like consistency, pour the white sauce into the frying pan with vegetables and fish. Stir thoroughly and leave to cook for another 3 minutes, stirring occasionally. Taste and season with salt and pepper if needed.
Prepare a large casserole (oven-proof) dish, transfer the fish and vegetable sauce into the casserole. Cover with the sliced potatoes, drizzle some olive oil on top and place in the oven. Bake for 30-35 mins or until slightly golden on top.
Serve with a rocket and pepper salad (optional). Enjoy!
Notes
Possible swaps:
use any fish that is available to you (boneless)
vegetables: use leeks, parsnip, broccoli or cauliflower instead of the veg in the recipe
semi-skimmed milk for oat or other plant-based milk