Heat oil in a skillet set over medium heat. Add onion and cook for 3 to 4 minutes until softened. Add diced chipotles, stir and incorporate refried beans and broth. Continue cooking until completely heated, seasoned and thickened to a soft puree consistency. Season with salt if need be.
In a bowl, combine the scallions, epazote or cilantro, queso fresco, pickled jalapeños, tomato and tomatillo. Taste for salt and add if need be. Set aside.
Top each tostada with a generous spread of the chipotle refried beans, a copious scoop of the seasoned queso fresco, and a few slices of avocado. Squeeze a bit of lime juice to taste, sprinkle with salt and black pepper.