Heat the oil in a large pan on medium/high heat. Cook the chopped onion and garlic until soft, about 1-2 minutes.
Place the fire roasted tomatoes in a small food processor (mine is 3 cups) and blend until well combined.
Stir the blended tomatoes, salsa, chili powder, cumin, paprika and red pepper flakes, if using, into the onion mixture. Season to taste with salt and pepper.
Take one tortilla and place 2 Tablespoons of the tomato sauce in a line along the bottom portion. Top with 1 Tablespoon of the thawed corn and 1/4 cup of shredded chicken. Roll up tightly and place in the bottom of a 7 quart slow cooker. Repeat with all tortillas - you should be able to fit them in one layer.
Pour the enchilada sauce on top and sprinkle with cheese. Place a paper towel on the top of the slow cooker and then place the lid on. Cook on low for 3-4 hours until the cheese is melted and the tortillas are soft