In a large pot, soak beans overnight, covered with several inches of water.
When ready, drain beans into a colander, then return to the pot, adding garlic, oregano, and 10 cups of water. Bring to a boil over high heat; stir and reduce heat to low, allowing the beans to simmer uncovered for 1½ to 2 hours, until soft.
Drain the beans in a colander over a large bowl, reserving all the cooking liquid.
In a large skillet over medium-high heat, melt bacon grease. Add onions and salt, cooking for about 10 minutes, until onions begin to brown.
Add jalapeños and chipotle pepper powder; cook for 1 minute.
Add the drained beans to the skillet, mashing half of them with a potato masher or the back of a spoon. Add ½ of the bean cooking liquid to the skillet, mashing and stirring the beans. Continue adding cooking liquid, mashing, and stirring the beans until they reach your desired consistency and texture.
Serve hot or cold, garnished with cotija cheese.