Preheat oven to 400°F.
Cut ½-inch off ends of plantain peels, slightly exposing plantain underneath. Insert tip of paring knife ½ inch into plantains and slit top to bottom, leaving peel in tact. Wrap plantains individually in 6 inch foil sheets. Place on baking sheet. Bake 30 minutes. Remove from oven, place plantains on plate, and set aside. Leave oven on.
Rub avocado oil on pepper, place on baking sheet. Bake 7 minutes or until skin chars. Remove from oven, place in bowl, and cover with plastic wrap 2 minutes or until steam loosens skin. Remove pepper from bowl, run under cold water, and rub lightly with fingers to remove charred skin. Dice pepper flesh into ¼ -inch pieces.
In small saucepan add chicken stock, shallots, garlic, poblano pepper, and guava paste, and bring to rolling simmer. Cook 5 minutes or until paste forms. Add rice vinegar, salt, and pepper. Reduce heat to low and cook 2 minutes, whisking continuously. Pour sauce in blender and purée until smooth.
Remove plantains from foil and cut each into quarters. Open plantains along slit without cutting in half. Divide quarters onto 8 plates, top each with 1 tablespoon sauce. Garnish with queso fresco and cilantro.