Preheat the oven to 200°C.
Fry the sliced onion and chopped garlic in vegetable oil until softened and stir in squid ink.
Add the paella rice and stir until every grain is evenly coated with the ink.
Pour in the fish stock, bring to a boil, then simmer for 15 minutes while stirring regularly.
Add the mussels and cook for 5 minutes, discarding any that remain closed.
Sift together the flour, celery salt, white pepper, and paprika onto a flat dish.
Melt the butter in an ovenproof frying pan over medium heat until it begins to foam.
Coat the monkfish pieces in the flour mixture and sear in the pan for 3 minutes while basting with butter.
Roast the monkfish in the oven for 5 minutes until cooked through.
Serve the monkfish over the rice and garnish with tomatoes, parsley, and lemon juice.