1. Melt the butter in a heavy-based pan and sauté the mushrooms until golden-brown. Stir in the green pepper, garlic and spring onions and fry for a few more minutes until the green pepper starts to soften. Stir in the flour and fry for 1 minute.
2. Using a balloon whisk, beat in the milk, stock and mustard and simmer until the sauce thickens. Stir in the tuna, check the seasoning and add salt and pepper if necessary.
3. Divide the filling between the pancakes, roll up and arrange in an ovenproof dish. Spoon any extra filling on top and sprinkle with the cheese. Place under a hot grill until golden-brown and heated through.