Preheat oven to 375˚F.
Lightly grease a 13x9-inch baking dish with cooking spray, add about 1/2 cup enchilada sauce to the bottom of the dish, and set aside.
Heat the olive oil over medium heat in a large skillet. Add onions and cook for 3 minutes. Stir in garlic and continue to cook for 20 seconds.
Add the shredded chicken, 1 cup enchilada sauce, 2/3 of the diced tomatoes, and sour cream; season with oregano, basil, salt, pepper, and chili powder.
Cook for 5 minutes, or until heated through.
Mix in half of the shredded cheese.
Spoon the chicken mixture down the center of each tortilla and roll it up.
Place the tortillas seam-side down in the baking dish.
Bake for 10 minutes.
Top with the rest of the enchilada sauce, remaining diced tomatoes, and shredded cheese.
Return to oven and bake for 10 minutes or until warm and the cheese is melted.