To make the essential topping of crispy tortilla strips, preheat the oven to 350F. Brush each tortilla generously with olive oil (you will use about 2 tablespoons total). Stack the tortillas four high and cut them into 3/4 inch strips with a sharp chef's knife. Scatter them around a rimmed baking sheet and bake for 20-22 minutes or until dark golden and crispy. Cool.
Preheat a broiler and place an oven rack on the highest level. Cut the onion into 6 wedges and toss in a cast iron pan. Quarter the tomatoes and add them to the pan as well, along with the whole garlic cloves. Place on the top rack of the oven and broil for 8-10 minutes, turning occasionally. The vegetables don't need to be cooked, just blistered and charred. Remove from broiler and cool slightly.
Carefully transfer the broiled vegetables to a blender along with any and all juices. Cover the blender with a lid and puree until smooth. In a large pot, heat the remaining 2 tablespoons of olive oil over medium heat. Carefully tip the pureed vegetables into the oil and cook for 10 minutes over medium low heat. Stir occasionally. Meanwhile, tip a little chicken stock into the blender to clean out every bit of goodness.
Pour all of the stock into the pot and add the diced zucchini. Bring to a simmer and cook for 8-10 minutes until the zucchini is tender (this is when I prep the rest of my toppings).
Tip in the beans, corn and chicken; stir to combine. Simmer for 3-4 minutes longer. Season with salt to taste. Your harvest tortilla soup base is now ready.
Prep your toppings: slice limes, dice the avocado, and finely chop the green onions. Place everything in bowls, along with the cilantro and the crispy tortilla strips. Serve up the soup in large bowls and present the toppings on the side. To enjoy, heap up avocado, green onion and cilantro onto the soup. Squeeze lime all over and top with a handful of crispy tortilla strips. Finish with a dollop of crème fraîche and enjoy.