Tip 1 tsp coriander seeds into a dry frying pan. Toast over a medium heat for 1-2 mins till the seeds start to pop and smell aromatic. Tip thm into a mortar and grind with a pestle, or use a small bowl and the bottom of a jam jar to roughly crush them. Set aside.
Peel and finely chop the onion. Trim and peel the carrot, then finely dice it. Trim the dry ends from the celery and finely slice.
Put the frying pan back on the heat. Add 1 tsp olive oil and the chopped veg. Season with salt and pepper. Cook over a medium-low heat, stirring occasionally, for 8 mins till the veg have softened.
While the veg sweat, peel and crush or grate the garlic. Finely chop the parsley stalks (keep the leaves for later).
Stir the garlic, parsley stalks and crushed coriander into the veg. Cook and stir for 1 min till the pan smells aromatic. Add the bay leaf and tip in the tin of tomatoes. Quarter fill the tomato tin with water (approximately 100ml) and add that to the pan. Pop on a lid and simmer for 15 mins, stirring a few times, till slightly thickened.
Preheat your grill to high. Finely grate half the block of Parmesan cheese. Roughly chop the parsley leaves.
Taste the tomato sauce and add a pinch more salt and pepper if you think it needs it. Divide the fish pie mix between 2 500ml heatproof dishes (or a 1 ltr dish). Pour the tomato sauce over the top, then scatter over the grated parmesan. Grill for 5-8 mins till golden and bubbling. Let the gratins cool for a few mins before serving with the salad leaves, garnished with the chopped parsley.