Break noodles into pieces and cook in plenty of boiling salted water just until al dente, drain, rinse under cold water and drain.
Peel onion and finely chop. Thaw peas. Cut ham into cubes.
Sweat onion in 1 tablespoon butter until tender, add peas, season with salt and pepper and cook for about 3 minutes. Remove from heat and stir in ham and half of the cheese.
Melt 2 tablespoons butter in a saucepan until foamy, stir in the flour, gradually pour in milk and simmer, stirring occasionally for about 5 minutes. Season with salt, pepper and nutmeg.
Mix the pasta with the onion mixture and pour into a buttered baking dish. Pour sauce over, sprinkle with remaining cheese and top evenly with remaining butter (cut into small pieces). Bake in an oven preheat to 180°C (approximately 350°F) center about 30 minutes until golden brown. Remove from oven and serve.