Place the onions (cut into eighths), whole tomatoes, and smashed garlic cloves on a large griddle or nonstick skillet over high heat.

Remove the garlic when it turns blackened. Turn the onions and tomatoes frequently and remove them when they are fully charred. Set aside.

Lightly toast the chiles and remove them when they change color. Set aside.

Using a stone mortar and pestle, add a little salt and the onion. Mash until almost smooth but with some pieces remaining. Transfer to a 1-quart bowl. Repeat with the charred tomatoes.

Mash the garlic into a paste. Add the chiles and continue mashing. Add this mixture to the other mashed ingredients. Season with salt and pepper to taste.

Take two freshly toasted banana leaf pieces and stack them on top of each other.

Place a cold corn tortilla on top. Add 2 tablespoons of the tomato sauce.

Season the chicken thighs with salt and pepper, then place one thigh on top of the tortilla and sauce.

Add another 2 tablespoons of sauce over the chicken. Add a few epazote leaves and cover with another cold corn tortilla.

Fold the first banana leaf piece to close like an envelope: first fold the side closest to you toward the center, then the top side so they overlap. Then fold the right side toward the center, and the left side the same way, overlapping.

Repeat with the second banana leaf piece, making sure it wraps the first one completely.

Wrap the tamal tightly in enough plastic wrap so that no part is left uncovered.

Steam for 1 1/2 hours over high heat. Add any leftover epazote leaves to the steaming water.

If you don't have a steamer, use a very large pot or Dutch oven. Place the water and epazote leaves at the bottom, then set a metal rack at least 1/2 inch above the water to hold the tamales. Line the bottom of the steamer or pot with leftover banana leaves or well-wrung, damp cotton kitchen towels.

Cover the pot with more well-dampened towels, a thick plastic bag, and more towels. The goal is to keep the steam from escaping easily.

Serve hot and enjoy with avocado guacamole.