Peel and finely chop the onion. Peel and grate or crush the garlic. Finely chop the parsley stalks and leaves, keeping them separate.
Warm 1 tbsp olive oil in a deep frying pan or wok. Add the onion. Season with salt and pepper. Fry over a low heat for 5 mins till the onion is just softened. Stir often.
Stir the garlic and parsley stalks into the onion. Add a pinch of the cayenne pepper (it's pretty spicy, so add as little or as much as you like). Cook and stir for 1 min till the pan smells aromatic and spicy.
Add the tomatoes to the pan. Half fill the can with water (around 200ml). Add that to the pan too. Stir in the olives. Cover. Bring the tomato sauce to the boil. Turn the heat down and simmer for 8-10 mins till the sauce starts to thicken.
Add the hake fillets to the pan, skin side down. Pop the lid back on. Simmer for 8-10 mins till the fish is opaque and cooked through.
Halve the lemon. Juice one half. Slice the other half into wedges for later. Pop the salad in a bowl and drizzle over the lemon juice. Season with salt and pepper. Toss to mix.
Taste the tomato sauce and adjust the seasoning if you think it needs it. Serve the hake and tomato sauce with the salad and lemon wedges, garnished with chopped parsley.