Place beans in a saucepan and add enough water to cover them by 1 inch. Bring to a boil and remove from heat. Let stand for 1 hour.

Drain and rinse beans. Add enough fresh water to cover by 1 inch. Bring to a boil, reduce heat, and simmer for 2 hours. Drain, reserving 1/2 cup of the cooking water, and rinse.

Heat oil in a skillet over medium-high heat. Add beans and salt. Fry until beans are very soft, stirring and mashing frequently, 5 to 7 minutes.

Set an oven rack about 6 inches from the heat source and preheat the oven's broiler on low heat.

Spread an equal amount of beans on each slice of bread and sprinkle queso on top. Place bread slices on a baking sheet.