Soak dried black beans in water for 6 hours or overnight.

Drain the soaked black beans.

In a 2-quart casserole dish, combine the drained black beans, pearl barley, diced sweet onion, and taco seasoning.

Add water to the casserole dish until it is approximately 1/2 inch above the level of the bean mixture.

Cover the casserole dish tightly with aluminum foil.

Bake in a preheated oven at 375°F (190°C) for about 3 hours.

After 1 hour of baking, check the water level and add more if needed (about 1/2 to 1 cup).

Check the water level again after 2 hours of baking and add more if needed (about 1/2 to 1 cup).

Alternatively, cook on the high setting of your slow cooker for 6-8 hours.

If desired, substitute dried pinto beans or great northern beans for the black beans.