TO MAKE THE FILLING: In a medium saucepan over medium heat, heat the oil and sauté the onions until just softened, about 3 minutes. Add the garlic and stir until fragrant, about 1 minute.
Add the epazote leaves, turn the heat to low, then add the black beans. Using a bean masher, potato masher, or back of a wooden spoon, smash the beans to form a chunky purée, adding in the cooking liquid, 1 tablespoon at a time, to loosen up the paste. Stir until the mixture comes together and is heated through. Season with salt.
TO MAKE THE MASA DOUGH: In a small bowl, combine all the ingredients and knead until a smooth dough is formed. Cover and allow to rest for at least 10 minutes.
TO MAKE THE TAMALES: Soak the corn husks in warm water for at least 3 hours, up to overnight. Drain the husks and separate them. Working with one at a time, remove a husk from the water and pat dry. Use your fingers to spread a handful of masa dough in the center of the husk, covering about two-thirds of the husk with masa about ¼ inch thick. Leave at least 2 inches on each edge clear.
Put 1 heaping tablespoon of the filling lengthwise down the center of the masa. Top with 1 tablespoon of queso fresco.
Wrap by bringing the right side of the dough over the filling and folding in half. Continue rolling tightly to the end of the husk, then secure one open end with string or strips of corn husk, or fold up toward the top. Repeat with the remaining corn husks and ingredients.
Prepare a large steamer by setting a steamer rack about 2 inches above gently boiling water.
Stack the tamales, seam-side down, on the rack. Cover and steam until the filling is firm and comes away easily from the husk, about 45 minutes. Remove the tamales with tongs. Serve warm with salsa and chile slices.
MASA RECIPE: In a large bowl, combine the corn flour, salt, and baking powder. Add the oil and use your hands to combine, until the mixture feels sandy.
Starting with 1 cup of water, pour a thin stream into the bowl, stirring constantly, then knead until incorporated; the dough should be tacky but not sticky. If needed, add more liquid, 2 tablespoons at a time. Form it into a ball. The dough can be sealed in a freezer bag and refrigerated for up to 3 days.