Mexican Black Beans (Frijoles Negros con Hoja de Aguacate)

Combine beans, garlic and water in a large pot. Bring to a boil over high heat. Reduce heat to maintain a gentle simmer and cook, stirring occasionally and adding more hot water if necessary to keep the beans just covered, until the beans are tender and creamy, 1 to 2 hours, depending on how fresh the beans are. Stir in salt. Remove from heat and let stand while you cook the onion.

Heat oil in a medium skillet over medium-high heat until shimmering. Add onion and cook, stirring occasionally, until starting to brown, 5 to 7 minutes. Reduce heat to medium-low and cook, stirring occasionally, until deep golden, 10 to 12 minutes more. Coarsely crumble avocado leaves (if using) into the pan and cook, stirring, until fragrant, about 30 seconds.

Puree 1 cup of the beans, 1/4 cup of the cooking liquid and the onion mixture (oil and all) in a blender. (Use caution when pureeing hot liquids.) Stir the blended mixture back into the beans. Swish a little more water around in the blender and add it to the beans too. Return to a simmer over medium heat and cook, stirring occasionally, until reheated, about 5 minutes.