Place the potato chunks and onion pieces in an even layer in a 6-quart slow cooker and pour in the 1/2 cup chicken broth.
Sprinkle half of the minced garlic, half of the oregano, half of the basil, half of the crushed rosemary, and season with salt and pepper over the potatoes and onions.
Heat 1 1/2 tablespoons olive oil in a large heavy-bottomed pot or skillet over medium-high heat.
Pat the skin of half the chicken thighs dry with paper towels and season both sides with salt and pepper.
Sear the seasoned thighs, skin side down, in the hot pot until the skin is golden brown, about 4 minutes; transfer to the slow cooker on top of the potato layer. Repeat with the remaining thighs.
Slowly and evenly pour 3 1/2 tablespoons fresh lemon juice over the chicken thighs.
Sprinkle the remaining minced garlic, remaining oregano, basil, and crushed rosemary over the chicken, then sprinkle 1 teaspoon lemon zest over the top (reserve the other teaspoon for serving).
Cover and cook on low for 5 1/2 to 7 hours, until the potatoes are tender and the chicken is cooked through.
Serve warm, spooning some of the slow cooker juices over each portion and sprinkling with chopped fresh parsley and the reserved 1 teaspoon lemon zest.