In a large bowl, use an electric hand mixer to mix the cream cheese, mayonnaise, sour cream, mustard, garlic, onion powder, salt, and pepper. Fold in 1 cup of mozzarella, ½ cup of Parmesan, and spinach, stirring until combined.
1 cup cream cheese, ½ cup mayonnaise, ½ cup sour cream, 1 tablespoon Dijon mustard, 2 cloves garlic, 1 teaspoon onion powder, 1 teaspoon kosher salt, ½ teaspoon ground black pepper, 2 cups freshly shredded mozzarella cheese, 1 cup freshly grated Parmesan cheese, 4 ounces fresh spinach
Pour and spread the mixture evenly on top of the chicken breast. Sprinkle the top with the remaining cheeses.
Bake for 25-30 minutes, until the casserole is warmed through and the cheese is golden brown and bubbly.
Broil the top for 2-3 minutes for a nice browned top.
Serve hot over rice, cauliflower rice, or pasta, if desired, or enjoy alone.
Notes
Be sure to fully drain the artichokes to avoid a watery casserole.
If using frozen spinach, let it thaw completely and drain off the excess liquid.
Storage: Store spinach artichoke chicken casserole in an airtight container in the refrigerator for up to 3 days or in the freezer for up to 3 months.