Remove the stems and seeds from the guajillo and arbol chiles. In a skillet over medium-low heat, toast them until they’re fragrant. This process releases their oils, resulting in a more flavorful salsa. Ensure you turn them often to prevent burning.
Place the toasted chiles in a pot, followed by the tomatillos and onions. Allow the mixture to simmer for 10-15 minutes, stopping once the tomatillos change color.
After draining the water, transfer the chiles, tomatillos, onions, garlic, and salt to a blender. Blend everything on high until the mixture is smooth. In a saucepan, heat some olive oil over medium. Pour the salsa in and simmer it to deepen the flavor, making sure to stir it often.
In a bowl, whisk together the masa harina and salt. Gradually add hot water and mix using a spatula. Once it’s cool enough to handle, knead the dough until the texture is moist but not excessively sticking to your hands.
Heat a cast-iron skillet or comal over medium, then portion the dough into balls. Add a spoonful of beans to the center, fold the dough over like an empanada, and shape the ends to achieve the distinctive tlacoyo design. Flatten it further using plastic wrap or wax paper.
Optionally, brush the skillet or comal with the flat part of an onion slice for an added layer of flavor. Cook the tlacoyos for a few minutes on each side, or until they’re firm and lightly charred. Happy eating!