Make the brine: Drain the pickle juice from the jar into a large bowl—you should have about 1 ¼ cups. Slice 4–5 of the pickles and set aside; save the rest for another time.
Brine the chicken: Add the dill, garlic, and chicken thighs to the pickle brine. Let sit at room temperature for at least 1 hour, or cover and refrigerate for up to 8 hours.
Cook the potatoes: Toward the end of the brining time, place potatoes in a large pot, cover with water, and season generously with salt. Bring to a boil and cook until fork-tender, about 15–18 minutes. Drain well.
Prep the chicken: Remove chicken from the brine, pat dry, and season lightly with salt and pepper.
Sear the chicken: In a large ovenproof skillet or Dutch oven, heat 2 tablespoons of the olive oil over medium-high heat. Add chicken, skin side down, and cook until golden, 6–8 minutes. Flip and cook 3 minutes more. Transfer chicken to a plate.
Roast everything together: Add the boiled potatoes to the skillet, gently smashing each one with the back of a spoon. Drizzle with ¼ cup olive oil and season with salt and pepper. Nestle chicken thighs among the potatoes.
Finish in the oven: Roast at 400°F until the chicken is cooked through (165°F internal temp) and potatoes are crispy around the edges, 25–30 minutes.
Serve: Scatter reserved pickle slices and more dill fronds over the top. Drizzle with the remaining olive oil and serve hot.
