Heat the butter in a large, nonstick frying pan, add the mushrooms, spring onions and garlic and stir-fry over a high heat for 6–7 minutes.
Stir in half of the cheese sauce and heat until just bubbling. Add the spinach and cook for 1 minute until just wilted. Set aside, stir in the chopped herbs and season.
Take 1 pancake and spoon one-eighth of the filling down the centre. Carefully roll the pancake up and put into a shallow, buttered gratin dish. Repeat with the remaining pancakes. Drizzle the remaining cheese sauce over the pancakes, sprinkle with grated Parmesan and season to taste. Cook under a preheated medium-high grill for 3–4 minutes, until piping hot and turning golden.
Remove from the heat and serve with lettuce leaves.