Heat your grill to its highest setting. Pour the milk into a large pan. Drop in a few peppercorns and add the haddock fillets. Bring to a gentle simmer, then take off the heat. Clamp on a lid and leave for 6 mins. Take the haddock out of the pan and pop on a warm plate. Keep the milk.
While the haddock sits in the milk, tip the tomatoes into a heatproof dish and place under the grill. Cook for 4-6 mins till they’re bursting from their skins. Finely chop the tarragon leaves and the chives.
Get a spotlessly clean, non-plastic bowl (this will help you whisk your egg whites). Separate your eggs, cracking the whites into this bowl and putting the yolks in another bowl.
Add half the tarragon and half the chives to the yolks. Add 2 tbsp of the haddocky milk and beat with a pinch of salt (see out tip on how to use up the rest of the milk). Set aside.
Whisk the egg whites till you have soft peaks that stand on their own when you lift the whisk out.
Gently fold the yolks through the whites, taking care not to beat out too much air. Heat a large frying pan. Add ½ tbsp olive oil. Spoon in half the soufflé omelette mixture and tip the pan so it covers the base. Cook for 3-4 mins till almost set and puffy.
Carefully slide the soufflé omelette onto a plate. Add another ½ tbsp olive oil and repeat to make a second omelette, sliding that onto a separate plate. Pop them both under the grill for 2 mins to cook the top. Turn the grill off and close it to keep the omelette warm while you finish the dish.
Tip the spinach into a pan. Zest and juice the lemon into it. Cook for 2-3 mins till it wilts. Add the crème fraîche. Heat gently for 2 mins. Flake the haddock into the pan and season. Stir gently to warm through. Top the omelettes with the creamy haddock and tomatoes. Garnish with the remaining tarragon and chives.