Heat the olive oil in a medium saucepan over medium heat until hot. Add the chopped red onion, oregano, basil, and garlic and cook over low heat until the onion is tender, stirring occasionally. Stir in the diced tomatoes, paprika, and salt and bring to a boil. Reduce heat to low, cover, and simmer for about 10 minutes. Remove from heat. Mix the sauce with the shredded chicken and set aside.
Melt 1 tablespoon of the butter in a large saucepan over medium heat. Add the sliced mushrooms and cook till water evaporates. Once the mushrooms turn brown, remove from skillet and set aside.
Return the empty skillet to the stove, add 3 tablespoons of the reserved butter and heat it over medium heat. Add the flour and stir the mixture for a minute. Gradually add the milk, stirring constantly over low heat until a smooth sauce is reached (about 5 minutes). Remove the pan from the heat. Pour one-third of the sauce into the blender with the spinach and blend until smooth. Pour the blender mixture back into the skillet with the remaining sauce. Stir to incorporate. Return to the medium heat, add the salt, pepper, and nutmeg. Stir in 1/2 cup grated Parmesan cheese and cook for about 2 minutes. Add the mushrooms. Taste the sauce to decide if it needs more salt. Remove from the heat.
Preheat oven to 375 F. Spray a 9 by 13-inch pan with nonstick spray. Arrange 3 noodles in bottom of dish. Cover with 1/2 spinach mixture. Place 3 more noodles over spinach mixture and cover them with 1/2 chicken mixture. Sprinkle the parmesan cheese evenly over the chicken. Cover with a layer of noodles and spread the remaining spinach sauce. Cover with a layer of noodles and spread with the remaining chicken mixture. Layer the chicken mixture with sliced tomato and sprinkle the top with the remaining Parmesan.
Cover the pan with aluminum foil and bake for 30 minutes. Remove the foil and continue baking for about 15 minutes, until the cheese is golden and the pasta is tender (test with a knife). Let the lasagna sit for about 10 minutes before slicing and serving. ENJOY!