Preheat oven to 500°F. Position a rack in the upper third of the oven.
Render bacon in a sauté pan until brown and crisp. Remove from pan, drain on paper towel and when cool, crumble. Set aside.
Place a large cast-iron baking dish or a rimmed baking sheet on the rack and allow to heat up.
Place the cut potatoes in a saucepan of heavily salted, boiling water. Cover with a lid, lower heat and cook the potatoes in gently boiling water until tender, about 15 minutes. Drain and toss them with the garlic and olive oil. (See Note 1)
Sprinkle the chicken thighs with the smoked paprika, remaining 1/2 teaspoon salt and 1/2 teaspoon pepper. Add 2 tablespoons parsley and the lemon juice. Toss to coat and set aside.
Carefully remove the hot baking dish from the oven. Carefully add the potatoes and spread in an even layer as this will sputter as it all hits the pan. Scatter the onions and 1 lemon cut into 8 pieces on top. Roast until the potatoes start to brown, about 12 minutes.
Turn the potatoes and lay the chicken pieces on top, adding any accumulated juices from the bowl. Return to oven and roast until the potatoes are tender and the chicken is cooked through, about 12 more minutes. Chicken should have an internal temperature of 165°F.
Remove from the oven and sprinkle sherry (optional) on top with the remaining 2 tablespoons parsley and crumbled bacon. Serve with the remaining lemon cut into wedges.
To cut the time down you can microwave the potatoes. Put the cut potatoes, garlic, olive oil, 1 tablespoon water and 1/2 teaspoon salt in a large microwave-safe baking dish and toss to coat. Cover with plastic wrap, pierce the plastic in a few places and microwave 8 minutes to cook partially.