Drain the canned salmon, discarding the liquid. Remove any skin and bones, if wished, then break the salmon into chunks.
Cook the rice in boiling water for about 15 to 20 minutes or until cooked, adding the vegetable stock powder to the cooking water to give the rice extra flavor.
Meanwhile, fry the bacon in a non-stick frying pan for 4 to 5 minutes, until crispy. Add the spring onions & mushrooms & cook for a further 4 to 5 minutes until softened. Mix with cooked rice, salmon chunks, herbs & seasoning.
Take eight large leaves from the head of cabbage & remove the thick stalk. Cook in boiling water until softened. Drain thoroughly. Lay the cabbage leaves on a work surface & divide the rice mixture between them, roll up leaves tightly (like a burrito).
Place a small amount of tomato sauce on the bottom of a 9 x 13-inch baking dish. Place rolls seam-side down in baking dish. Top with remaining tomato sauce. Cover with foil & bake for 10 to 15 minutes, until piping hot.
Remove from oven & sprinkle the grated cheese over the cabbage rolls. Return to oven for another 5 minutes or until cheese has melted. Remove from oven & serve with crusty rolls.