2 C. shredded cheese (I used a combination of parmesan, monterey jack - see note)
Opt: ½ C. shredded mozzarella cheese
Preheat oven to 350ºF
Cook pasta al dente according to package directions. Drain. Return to pan and toss with a bit of olive oil to keep pasta from sticking together.
While pasta is cooking, slice chicken tenders into strips about ¼-1/2" thick. Season pieces with salt and pepper.
In large skillet, heat a little olive oil to coat bottom of pan. Add chicken strips and sauté until no longer pink. Remove chicken from skillet and set aside.
Add onions, mushrooms and garlic to pan (add a little more olive oil if needed). Saute until onions and mushrooms are tender being careful not to burn the garlic.
Add spinach and toss to combine. Cook until wilted. Add chicken back to pan and stir to combine. Add chicken mixture to pasta and toss to combine.
In saucepan, heat butter over medium heat until melted. Add flour. Whisk and continue to cook. about a minute. Slowly add milk, while continuing to whisk. Cook until sauce has thickened. Remove from heat and stir in 2 cups cheese and pesto.
Add sauce to pasta/chicken mixture and toss well to combine.
If you want extra cheesiness, layer half of pasta mixture in greased casserole dish. Sprinkle ¼ c. shredded mozzarella cheese over top. Add remaining pasta/chicken mixture. Top with remaining mozzarella. I also added a little more parmesan on top. You can omit this cheese layer if you want but why?
Bake until heated through and lightly golden on top, about 20-25 minutes.