Juice one of the limes and cut the other into wedges to serve with the soup.
Heat a drizzle of oil in a medium pot and sauté the onion for 3-4 minutes, until soft.
Add the garlic and jalapeno and cook for another minute.
Add the chicken, stock, lime juice and oregano. Season with salt and pepper to taste and bring to a simmer; reduce the heat and cook for 10-12 minutes.
Meanwhile, heat about half an inch oil in a small skillet and cook the tortilla strips until golden and crisp. Drain on paper towels.
Ladle the soup into bowls and top with the crispy tortilla strips, crumbled feta and avocado.