Place potato cubes in a small saucepan, and cover with water. Place saucepan over medium-high heat and bring to a boil. Cook potatoes until tender when pierced with a fork, about 10 minutes. Drain potato, reserving 1 cup of cooking liquid.
While potato boils, heat oil in a large saucepan over medium-high heat. Sauté onion until golden brown and beginning to caramelize, about 15-20 minutes. Add salt, and pepper, 3/4 cup reserved cooking liquid, tomato paste, chipotle paste, and sugar; add white beans and cooked potato. Cook, stirring, until bubbling, about 2 minutes. Stir in more reserved cooking liquid, if needed to make it saucy.
Heat tortillas in a dry skillet until toasted and browned. Set tortillas on plates. Top with white bean potato filling. Garnish with cabbage, salsa, queso fresco, cilantro, and avocado. Drizzle with sour cream.