Blanch one third of the escargots in boiling water for less than a minute. Use a toothpick to pick out the meat. Cut into small pieces and put aside. Blanch bacon in boiling water and drain well in a colander. Dice the bacon and fry in olive oil until fragrant.
Heat butter in a non-stick saucepan, fry shallots and garlic until lightly golden and fragrant. Add flour, salt, sugar, pepper and freshly cracked black pepper. Keep stirring over a gentle low heat, then mix in water and milk until the mixture turns smooth.
Add chopped escargot meat, bacon and button mushrooms. Combine well then transfer to a baking pan. Sprinkle with mozzarella cheese and surround the dish with the remaining escargot. Bake in a preheated oven at 180°C for 10-15 minutes or until the top is lightly golden.