Rinse lentils well under cold running water. Drain and transfer to a slow cooker. Pour in water until water level is twice as high as lentils. Add bay leaf and set slow cooker to High.

Meanwhile, heat onion and bacon in a skillet over medium heat. Cook until bacon is crispy, 5 to 10 minutes. Season with garlic powder and transfer bacon-onion mixture to the slow cooker.

Cook on High until lentils are tender, about 5 hours.