For the black beans: Heat the oil in a medium saucepan over medium heat. Add the onions and cook until soft, about 3 minutes. Add the garlic and cook 30 seconds. Add the beans, 1/2 cup water, and the chipotle and cook, stirring occasionally, until the water is reduced and beans are heated through and soft, about 20 minutes. Use a potato masher to mash the beans into a chunky paste. Taste, and season with salt as necessary. Stir in the cilantro, and then spread on a baking sheet to cool.
For the salsa: Put the avocados, tomatoes, green onions, serrano and some salt and pepper in a large bowl and fold together. Fold in the lime juice, oil, cilantro and watercress. Set aside.
For the chorizo filling: Heat the oil in a large skillet until it begins to shimmer. Add the chorizo and cook until golden brown and cooked through, about 8 minutes, stirring a few times. With a slotted spoon, remove the sausage to a paper-towel-lined plate.
Add the onions and peppers to the pan drippings and cook, stirring occasionally, until soft, about 8 minutes. If the chorizo is in large chunks, chop it coarsely, then stir it into the onions and peppers. Add the eggs, and scramble until soft curds form. Season with salt and pepper, then stir in the cilantro and remove from the heat.
To assemble: Preheat the oven to 425 degrees F. Brush a nonstick baking sheet with some oil. Spread a thin layer of the beans across the center of a tortilla, leaving a 1-inch border. Top with some of the chorizo filling and a handful of Monterey Jack. Fold in the sides of the tortilla, and then roll from the bottom into a tight cylinder. Place seam-side down on the baking sheet and repeat with remaining tortillas, beans, filling and cheese. Brush the tops and sides of the burritos with canola oil, and bake until golden brown and crispy, about 7 minutes. Transfer to a platter, and top with Avocado-Tomato-Watercress Salsa, cotija and cilantro leaves. Garnish the platter with lime wedges.