In a medium saucepan, heat oil over medium heat until hot. Add 1/4 cup of the onion and the chile de arbol; cook, stirring constantly, until onions begin to brown, about 5 minutes. Add chicken broth, bring to a boil; reduce heat to low and simmer, covered, 5 minutes. Meanwhile, in a medium bowl, coarsely mash the avocados. Stir in cheese, remaining 1/2 cup onion, the cilantro, Serrano chile, salt and pepper. Heat a heavy skillet over medium heat until hot; add a tortilla and warm 30 to 60 seconds on each side. Remove tortilla to work surface and spoon about 1/3 cup avocado filling onto center in a crosswise mound. Roll sides of tortilla over filling and set aside fold side down. Repeat to use all of filling and make 8 rolls. Remove chile from chicken broth. In a blender, combine beans and broth; blend until smooth. Return to saucepan and bring just to a boil, about 5 minutes. Set aside 1/2 cup bean sauce. Divide remaining sauce among 4 plates. Place 2 rolls on the sauce on each plate; drizzle remaining sauce over rolls. Top with onion, crumbled cheese, and chopped cilantro, if desired. Serve immediately.