Preheat oven to 350°F and grease a 9x13-inch baking dish with nonstick spray. Set aside.

Heat olive oil in a large skillet over medium-high heat. Sauté onions until translucent, about 4 minutes. Add garlic and cook for 30 seconds.

Add spinach and chopped artichokes and cook until spinach wilts. Remove mixture to a large bowl and set aside.

In the same skillet, melt butter. Whisk in flour and cook for 1 minute, stirring frequently to form a roux.

Gradually whisk in warm milk, about 1/2 cup at a time, stirring constantly until thickened and smooth.

Stir in Italian seasoning, Monterey Jack cheese, salt, and pepper. Mix until cheese is melted and sauce is smooth.

Add sauce to the spinach-artichoke mixture along with the shredded chicken. Mix to combine.

Transfer mixture to prepared baking dish, top with parmesan cheese, and bake for 30 minutes or until bubbly and golden brown.

Let rest for 5 minutes before serving.