Preheat oven to 375°F (190°C). Oil four 6.25-inch cast iron skillets. Arrange on a baking sheet and place in oven to preheat.
Heat vegetable oil in a saucepan over medium heat. Add onion and garlic and cook for 3 minutes or until softened and fragrant. Add tomatoes and their juices along with oregano, chipotle, and salt. Add 1/4 cup chicken broth and simmer for 5 minutes. Let cool slightly, then transfer to a blender and blend until smooth. (Sauce can be made up to 2-3 days ahead of time; rewarm prior to using).
Return saucepan to medium heat (no need to wash it out). Add refried beans and remaining 1/4 cup chicken broth and stir until smooth and warmed through.
Remove baking sheet with skillets from oven. Carefully (skillets will be very hot) press a tortilla into the bottom of each skillet. Spread 1/4 cup refried beans onto the tortilla, making a small ‘well’ or indentation in the center to hold the egg. Sprinkle with 1/4 cup shredded cheese, and top with a few tablespoons of sauce. Repeat with all four skillets.
Gently crack one egg into the center indentation of each skillet. Return baking sheet to oven and bake for 10 minutes or until egg whites are starting to turn opaque; yolks will still be quite jiggly. Sprinkle an additional tablespoon or two of cheese around the yolk, then return to oven for another 5 to 7 minutes, until cheese is melted and egg is cooked to desired doneness (ideally the whites will be set but the yolk will still be slightly runny).
Carefully (remember skillets are very hot!) release tortilla from bottom of skillet with a thin spatula, and slide the whole thing out onto a plate. Top with sliced avocado and fresh cilantro. Serve warm with additional sauce as desired.