Wash the salmon, dab dry, dice. Cover and freeze for about 30 minutes. Wash tomatoes, peel onion. Dice finely and stew in hot oil for about 5 minutes. Season with salt and pepper.
Melt butter for the béchamel sauce. Briefly sauté the flour in it lightly while stirring. 1⁄8 Add l water and milk while stirring, bring to the boil and simmer for about 5 minutes. Season to taste with stock, salt, pepper and lemon juice.
Mix salmon, sour cream and 1-2 tablespoons of ice-cold water (to prevent the farce from curdling) in a mixer to a smooth farce. Wash the dill and chop finely. Stir in all but 1 tbsp. Season the farce with about 1/2 tsp salt and pepper.
Spread some béchamel in an angular casserole dish. Pour the farce into a bag, cut off one corner. Place the cannelloni vertically on the work surface. Spray the farce into the cannelloni.
Place the cannelloni close together in the mould. Pour the rest of the béchamel sauce over it. Preheat the oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3).
Cut the mozzarella into thin slices and place them on the cannelloni. Stir the rest of the dill into the tomato sauce and spread on the cheese. Bake cannelloni in a hot oven for 40-45 minutes.