Preheat oven to 350ºF. Coat a large, deep lasagna pan with cooking spray. Measure 2 cups of marinara sauce; set aside.
Heat oil in a large skillet over medium-high heat. Add onion; cook and stir until almost tender, about 3 to 4 minutes. Add chicken; cook until no longer pink, stirring frequently, about 4 to 5 minutes. Add remaining marinara sauce to skillet and stir until well combined; set aside.
Mix together ricotta cheese, egg whites, garlic powder and Italian seasoning in a large bowl until well combined.
To assemble lasagna, place 2 cups of reserved marinara sauce on bottom of dish. Layer with 3 noodles, 1/3 of ricotta mixture, 1/3 of spinach, 1/3 of mozzarella cheese and 1/3 of chicken-sauce mixture; repeat layers ending with chicken-sauce mixture. (Make sure to spread ingredients to edge of noodles.)
Cover baking dish with aluminum foil. Bake lasagna for 20 minutes. Uncover; sprinkle with remaining mozzarella cheese and bake until cheese is melted and edges are bubbly, about 20 minutes more. Remove from oven and let stand 10 minutes before slicing into 8 pieces.