Step 1

Place chicken and 1/4 onion in a large pot. Add 8 cups water and 2 teaspoons salt. Bring to a boil and skim off any foam from the surface. Reduce heat to a gentle simmer, cover and cook until the chicken is just cooked through but not yet pulling away from the bone, about 20 minutes. Remove the chicken to a bowl; discard the onion. Reserve the broth.

Step 2

Meanwhile, place corn, potatoes, carrots and squash in a 10- to 12-quart stockpot.

Step 3

Set aside 1/4 cup broth. Add enough water to the remaining broth to equal 10 cups and add to the stockpot with the vegetables. Add the remaining 2 teaspoons salt. Cover and bring to a boil over high heat. Add green beans. Reduce heat to maintain a simmer; cover and cook until the vegetables are slightly tender but not completely cooked, 8 to 10 minutes. Add zucchini. Remove from heat and add the chicken; cover to keep warm.

Step 4

Heat oil in a large skillet over medium heat. Add diced onion and cook, stirring often, until translucent, 4 to 6 minutes. Add tomatoes and cook until softened, about 3 minutes. Stir in mushrooms and the reserved 1/4 cup broth; bring to a simmer. Reduce heat to medium-low, cover and cook, stirring occasionally, until the mushrooms are tender, about 10 minutes. Stir in chipotles. Transfer the mixture to the stockpot and stir to combine. Add epazote leaves (if using). Cover and cook the soup over medium-low heat until the vegetables are tender, about 10 minutes more. Serve the soup in large deep bowls.