Preheat oven to 350 degrees F (175 degrees C).
Bring a large pot of lightly salted water to a boil. Cook lasagna noodles in boiling water for 8 to 10 minutes. Drain, and rinse with cold water.
In a large saucepan, melt butter over medium heat. Cook onion and garlic in butter until tender, stirring frequently. Stir in flour and salt; simmer until bubbly. Mix in the milk and chicken broth, a little at a time, stirring constantly. Bring to a boil and cook for 2 minutes. Remove from heat and stir in 2 cups mozzarella cheese and 1/2 cup Parmesan cheese. Season with basil, oregano, thyme, parsley, tarragon, paprika, and cayenne pepper.
Layer 1/3 of the lasagna noodles, 1/2 of the shrimp and 1/2 of the crabmeat, 1/2 of the spinach, and 1/3 of the sauce mixture in an 11x7 inch baking dish. Repeat layers, and top with remaining noodles and sauce. Sprinkle with remaining mozzarella and Parmesan cheese. Bake uncovered for 40 minutes. Let stand 10 minutes before serving.