Preheat the oven to 425°F. Spread 1 teaspoon olive oil on a large sheet pan or lightly spray with nonstick spray.
Place chopsticks or wooden spoons on both sides of one chicken breast (on the long sides) and make cuts across the chicken about a half inch apart, without cutting all the way through. Place chicken on prepared sheet pan.
In a large skillet, heat 1 teaspoon olive oil over medium heat. Add onion, salt and pepper; cook, stirring, until translucent, or about five minutes. Chop ½ cup of the drained artichoke hearts and add them to the onions (reserve remaining artichoke hearts). Add the spinach and cover. Cook 2-3 minutes or until the spinach is wilted, stirring occasionally as needed,
Once the spinach is wilted, add cream cheese, mozzarella cheese, and parmesan cheese. Stir until the cheese melts and is combined. Remove from heat.
Using a teaspoon, carefully place spinach mixture into each one of the slices you made in the chicken.
In a bowl, combine the remaining artichoke hearts and halved Roma tomatoes with one tablespoon olive oil until vegetables are coated. Arrange the vegetables around the chicken breasts in a single layer; season with black pepper.
Bake for 15 minutes or until internal temperature of chicken is 165°F (insert instant read thermometer into thickest part of chicken). Serve immediately.
Baked chicken can be refrigerated and stored for up to three days. To reheat, microwave until heated through.