Prep your bird precisely: Remove chicken from refrigeration 30 minutes before cooking—room-temperature meat cooks evenly throughout. Pat completely dry with paper towels, removing all surface moisture. This prevents steaming and encourages golden, crispy skin development during roasting.

Build the herb marinade: Combine olive oil, lemon juice, minced garlic, chopped rosemary, chopped thyme, sea salt, black pepper, and paprika in a small bowl. Whisk thoroughly until garlic suspends evenly throughout the mixture. Reserve 2 tablespoons of this marinade separately for the pan sauce later.

Season inside and outside: Rub three-quarters of the herb mixture all over your chicken's exterior, working under the skin at the breasts and thighs. Stuff the cavity with lemon slices and onion quarters to perfume the meat from within. This aromatic cavity infusion ensures the herb flavor permeates every bite of juicy lemon herb chicken.

Roast at high heat: Place chicken breast-side up in a roasting pan. Roast at 425°F for 45 minutes, or until the internal temperature reaches 165°F at the thickest thigh. The skin will turn deep golden-brown; tent loosely with foil if browning too quickly after 30 minutes.

Rest before carving: Remove from oven and let the chicken rest undisturbed for 10 minutes in the pan. Resting allows carryover cooking to finish interior doneness while juices redistribute throughout muscle fibers. This step prevents dry chicken—it's non-negotiable for tender results.

Build the pan sauce: Pour pan drippings into a saucepan over medium heat. Add reserved herb marinade and chicken broth, scraping browned bits from the pan bottom. Whisk in cold butter and Dijon mustard until emulsified and glossy. Simmer 2 minutes; taste and adjust seasoning with salt and pepper.

Carve and serve: Transfer rested chicken to a cutting board. Separate thighs, drumsticks, and wings cleanly. Slice breasts against the grain into half-inch pieces. Arrange on a platter and drizzle generously with the pan sauce.