Preheat oven to 425°F
Place spinach in a lightly sprayed 9x13" baking dish. Sprinkle with 1/4 teaspoon Kosher salt and 1/4 teaspoon pepper.
Heat olive oil in a large skillet over medium heat. Add the chicken tenderloins, and cook until no longer pink inside, about 8-10 minutes (turning over halfway through). Remove chicken from skillet and place over spinach.
In a clean saucepan, melt the butter. Stir in the flour until smooth; gradually add milk. Bring to a boil; cook and stir for 1-2 minutes until thickened.
Stir in Italian Seasoning, cheddar cheese, remaining salt and pepper. Stir just until cheese is melted and mixture is smooth. Add the diced tomatoes and cook for an additional 1 minute.
Pour the sauce over the chicken and spinach. Bake for 20 minutes, or until sauce is bubbly.
Serve alone or over pasta or rice.