Step 1Spray a 4-quart saucepot with nonstick cooking spray, add the oil and heat over medium heat. Add the onions and garlic and cook over medium-low heat until the onions are tender but not browned, about 5 minutes.
Step 2Stir in the cubed chicken. Cook and stir until the chicken turns white and is cooked on the outside, about 5 minutes. Stir in the oregano and cumin. Stir in the chicken broth and beans. Heat to boiling. Reduce the heat to a simmer and cook about 10 minutes.
Step 3Blend together the cornstarch and 3 tablespoons of the water until smooth. Stir the mixture into the simmering chili until blended. Return the chili to a simmer and cook until thickened and clear, about 5 minutes. Blend together the masa and the remaining 3 tablespoons of water; stir into the chili to thicken. Stir in the spinach. Bring to a boil and simmer 1 minute. Stir in the vinegar. Season to taste with salt and pepper.
Step 4Serve the chili over the baked potatoes. Garnish with the cilantro, green onions, lime wedges and sour cream.