Cook lasagne noodles as directed. Drain and rinse with hot water.
In a large saucepan, melt butter. Add mushrooms, garlic, salt and lemon juice. Saute mushrooms until tender, about 5 minutes. Stir in flour and chicken boullion; blend well. Gradually add milk and cook over medium-high heat until mixture thickens and boils, stirring constantly. Stir in chicken and 1/3 cup of parsley.
Heat oven to 325°F.
To assemble lasagne, spread 1/2 cup of sauce in an ungreased 13 x 9" baking dish. Layer 1/3 of the noodles, 1/3 of the ricotta, 1/3 of the mozzarella, 1/3 of the sauce and 1/3 of the parmesan cheese. Repeat layers ending with parmesan on top. Sprinkle with remaining parsley.
Bake for 45 minutes or until bubbly. Let stand 10 - 15 mintues before serving.