Blend the diced tomatoes, garlic and onion powder in a blender until smooth, then pour into a large Dutch oven and cook on medium-high heat until the liquid has reduced to half, about 5 minutes.Stir in the chicken broth, chicken and corn, and then season with the cumin and chili powder. Taste the soup and then season with salt and pepper to taste.Combine the 3 teaspoons of cornstarch with 1 tablespoon of water and then stir into the soup. Bring the soup to a boil and then reduce the heat and simmer for 20 minutes, stirring occasionally.Divide the tortilla strips among the serving bowls, ladle in the soup and top with cheese and any other optional garnish you desire.