Combine the olives, lemon zest and garlic in a small bowl. Set aside.
Mix the smoked paprika, olive oil, 3/4 teaspoon of the salt and pepper into a paste in a large bowl.
Lay the chicken thighs skin-side down. Using a pair of kitchen shears, make a cut down the length of the bone to expose it, then cut the meat away from the bone. Discard the bone.
Stuff about 2 tablespoons of the olive mixture under the skin of each chicken thigh and rub paprika paste on each. Arrange the chicken thighs, skin-side up, on a cooling rack.
Arrange the potato slices and onion pieces in an even layer on a foil-lined half sheet pan. (I also sprinkled left over paste on potatoes.).
Place cooling rack over potatoes and bake until the skin is crispy and the potatoes are tender, 55 to 60 minutes. If you prefer the potatoes crispy, remove the rack with the chicken and return to the oven for an additional 5 to 10 minutes. Serve immediately.