Preheat oven to 375 degrees. Trim the chicken breasts and use your hand to flatten out the thick end of the breast a bit. Lay them in a 9x13 baking pan that has been lightly greased.

In a skillet, heat the oil over high heat and then add the onions and mushrooms. Cook until they are softened.

In a large microwave safe bowl, place the cream cheese and chicken broth. Microwave on high for 1 1/2 minutes and then use a whisk to blend completely. Add the dressing mix, garlic powder, pepper and salt. Mix well. Mix in the cooked onions and mushrooms, along with the artichokes, tomatoes and spinach.

Pour the mixture over the chicken and bake for 35-40 minutes, until the chicken is firm. Add the cheese over the top and place under the broiler to brown the cheese.