Boil Pasta: Bring 8 cups of water with 1 ½ tsp salt and 2 chicken stock cubes to a boil. Add in the lasagna sheets and cook until al dente according to package instructions.
Prepare Sauce: On medium heat, melt butter. Add red bell pepper and onion, cooking for 4 minutes. Add garlic and cook for 30 seconds before adding spinach. Cook for 2 minutes.
Mix Ingredients: Add in flour and cook for 2-3 minutes. Stir in reserved pasta water, half and half, and cheese. Season with salt, pepper, and Italian seasoning. Mix in rotisserie chicken and cook for another 2 minutes. Set aside.
Assemble: Layer the lasagna with sauce, then 3 lasagna sheets, sauce, and ½ cup mozzarella. Repeat layers and finish with 1 cup mozzarella on top.
Bake: Bake at 350°F for 30 minutes. Broil on low for 6-7 minutes. Garnish with parsley.