I suppose I should mention that when I make this soup, I also make homemade Mexican rice at the same time. But if you already have Mexican rice, you can use that. Don't skip the rice, though!
First, chop up 2 of the onions in largish chunks. I suppose you can dice it small if you don't like onions, but I love them, so I do chunks. Mince the garlic.
Pour about 1 cup of the broth into a large soup pot and kind of saute the onions and garlic.
Meanwhile, cut the carrots lengthwise into matchsticks. I think the shape is important. I don't think they taste right in coins. If you haven't tried matchsticks, now is the time.
Add the rest of the broth and the carrots and bring to a boil. Boil until the carrots are just starting to get soft. I don't exactly know how long this takes, so check every couple minutes by sampling one.
When the carrots are starting to get soft, turn it down to simmer and add the tomato sauce. Give it a little sprinkle of salt and a squirt of lime juice. Add the black beans and corn and simmer for a few minutes.
Add chipotle chilies along with a few spoonfuls of adobo sauce. Stir it around a bit and then remove the chilies. Don't worry if it's not super spicy, because it will get spicier as it sits.
Sometimes I add water to get it soupier. It should be soupy.
Now take your last onion and dice it very finely. Put it in a bowl, this is like a condiment. Put the rice in another bowl, this is also a condiment. Cut up some limes into wedges, warm the tortillas and put them in something to keep them hot. Like a tortilla keeper or maybe a clean dishtowel. Slice the avocado into thin slices and put them on a plate, this is a condiment too.
To serve: put the soup in bowls. Spoon as many fresh onions and rice as you want into the bowl. Squeeze some lime juice in. And then: this part here is key -- this is what makes the whole thing come together-- put 1 to 2 avocado slices into a hot tortilla, sprinkle with salt, squeeze lime over the top, roll up, kind of mush the whole thing so the avocado gets a little squished in there and then dip in the hot soup. Glorious.