Preheat oven to 350 degrees then coat a 2-quart casserole dish with cooking spray. Place a large pot of water over high heat then bring to a boil. When the water comes to a boil, add a generous pinch of salt and pasta. Cook, stirring occasionally for about 2 minutes less than the recommended cooking time on the package. Drain completely, transfer pasta to a large mixing bowl then set aside. Leave colander in the sink to drain the spinach.
Place large skillet over medium heat, add spinach and ¼ cup of water. Cook, stirring frequently, just until the spinach is wilted and becomes dark green. Remove spinach from pan then place in the strainer. Press as much water as possible from the spinach by pressing with the back of a spoon then set aside to cool.
Return skillet to heat then add olive oil. When the oil is hot, add cubed chicken then cook for 2 minutes until chicken begins to turn white. Add salt, pepper, ¼ tsp garlic powder, onion powder, dried basil and dried oregano then stir to combine.
Cook for 3-4 minutes or until the chicken becomes light golden brown and is barely cooked through. Don’t worry if the chicken is slightly undercooked—it will finish in the oven. Transfer chicken from skillet onto a plate then set aside. Leave remaining oil in the skillet, and if necessary, add enough additional olive oil to make 2 Tbsp. Add onions and cook 3-4 minutes until softened.
Add 2 Tbsp of flour to oil in the skillet then stir to dissolve. Cook for 1 minute then stir in milk to dissolve flour mixture completely. Bring milk to boil then let it bubble for 5 minutes, stirring constantly. Reduce heat to medium-low then add cream cheese. Stir until completely melted and incorporated then add asiago cheese, garlic powder, salt and pepper. Remove from heat then stir to combine.
Chop spinach coarsely then stir it into the sauce. Once spinach is distributed through the sauce, pour sauce over the pasta in mixing bowl. Add chicken to the bowl then toss to combine. Transfer mixture to the prepared casserole dish then top with mozzarella cheese. Bake for 25–30 minutes or until the cheese is melted and golden brown and the sauce is bubbly. Enjoy!
One Last Thing
In the off chance you have leftovers—add a bit of milk when reheating to retain the moisture in the sauce.