Preheat oven to 180 C / Gas 4. Bring a large pot of lightly salted water to the boil. Cook lasagne sheets in boiling water for 8 to 10 minutes. Drain, and rinse with cold water.
Melt the butter in a large saucepan over medium heat. Cook the onion and garlic in the butter until tender, stirring frequently. Stir in the flour and salt, and simmer until bubbly. Mix in the stock and milk, and boil, stirring constantly, for 1 minute. Stir in 1/2 of the mozzarella cheese and 1/4 of the Parmesan cheese. Season with the basil, oregano and black pepper. Remove from heat, and set aside.
Spread 1/3 of the sauce mixture in the bottom of a 22x33cm baking dish. Layer with 1/3 of the lasagne sheets, the ricotta and the chicken. Arrange 1/3 of the lasagne sheets over the chicken, and layer with 1/3 of the sauce mixture, spinach and the remaining mozzarella cheese and Parmesan cheese. Arrange remaining lasagne sheets over cheese, and spread remaining sauce evenly over sheets. Sprinkle with parsley and 4 tbsp Parmesan cheese.