In a slow-cooker, toss onion, potatoes, and lemon juice together with olive oil. Spread into an even layer.
In a large bowl, stir together salt, pepper, and garlic powder. Add chicken breasts and toss to coat evenly.
Place chicken in a single layer above the onion mixture and top with rosemary.
Cook on low until chicken is cooked through and potatoes are fork-tender, 4 to 5 hours.
