Preheat oven to 400 degrees. Cook the rice according to package directions. Keep covered while you make the soup.
In a Dutch oven or medium pot, heat 1 tbsp. oil over medium-high. Add the onion and cook, stirring occasionally, until lightly browned, about 7 minutes. Add the stock, tomatoes and crushed red pepper; bring to a boil over high. Stir in the chicken and cooked rice; season with salt and pepper. Reduce heat to medium-low; simmer 10 minutes.
Meanwhile, brush the tortillas with the remaining 2 tsp. oil; stack, then slice into 12 strips each. On two baking sheets, arrange the strips in a single layer. Bake, rotating baking sheets halfway, until crispy, about 12 minutes.
Zest and juice 1 lime; stir into the soup. Top with the tortilla strips and avocado. Cut the remaining 1/2 lime into 4 wedges; serve with the soup.
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