Prep the oven and pan. Heat the oven to 425°F (220°C).
Line a sheet pan with foil for easy cleanup and top with a wire rack, or use a large oven-safe skillet.
Mix the sauce. In a bowl, whisk maple syrup, Dijon, vinegar or lemon juice, garlic, olive oil, thyme, smoked paprika, red pepper flakes (if using), 1 teaspoon salt, and 1/2 teaspoon pepper until smooth.
Season the chicken. Pat thighs dry with paper towels. Sprinkle both sides with salt and pepper. Dry skin equals better browning.
Optional quick marinade. If you have time, toss the chicken in half the sauce and let it sit 20–30 minutes.
Reserve the other half for basting. If not, move on.
For extra-crispy skin (stovetop start): Heat a film of oil in a large oven-safe skillet over medium-high. Place thighs skin side down and sear 5–7 minutes until deep golden.
Flip, brush with sauce, then transfer the skillet to the oven.
For sheet-pan roasting: Arrange thighs skin side up on the rack. Brush generously with sauce, keeping some for basting.
Roast to perfection. Cook 25–35 minutes, basting once or twice with more sauce, until an instant-read thermometer reads 175–190°F in the thickest part and the skin is caramelized.
Finish the glaze. Pour any remaining sauce into a small saucepan. Simmer 2–3 minutes until slightly thickened.
Whisk in butter for a glossy finish.
Rest and serve. Let the chicken rest 5 minutes. Spoon over the warm glaze, hit with lemon juice if you like, and sprinkle with parsley.