Wash the fish fillet, drain well and dab with kitchen paper. Sprinkle with lemon juice and cover and let it stand for a short time. Peel onion and garlic. Cut onion into small cubes and crush garlic finely.
Clean the tomatoes, boil them with boiling water and skin them. Chop the tomatoes. Heat fat in a pan. Steam fish in it for 5-7 minutes, covered. In the meantime wash basil and remove leaves.
Chop basil leaves finely and grind them with parmesan, half of the garlic, oil and pine nuts with a mortar or puree them in the universal chopper. Season to taste with salt and pepper. Season fish and keep it warm for a short time.
Steam the remaining finely crushed garlic and tomatoes in the fish stock for 2 minutes. Season to taste with salt and pepper. Serve with the fish on a preheated plate. Serve with crispy baguette.