I can't tell you how many times I've made this! I have the butcher butterfly the chicken so all I do is grind the thyme, fennel seeds, salt, and pepper, mix it with olive oil, and brush it on the chicken. When the lemon slices are roasted and caramelized, you can eat them with the chicken.
Notes: Remove the ends of the lemon, cut in half through the stem ends, and slice thinly crosswise. Sometimes I sprinkle the chicken with minced fresh rosemary before allowing it to rest.
Make-ahead tip: Assemble the chicken in the pan and refrigerate for a few hours before roasting.
Preheat the oven to 450ºF.
Place the thyme, fennel seeds, 1 tablespoon salt, and 1 teaspoon pepper in a mini food processor and process until ground. Pour the olive oil into a small glass measuring cup, stir in the herb mixture, and set aside.
Distribute the lemon slices in a 12-inch cast-iron skillet and distribute the onion and garlic on top. Place the chicken, skin side down, on top of the onion and brush with about half the oil and herb mixture. Turn the chicken skin side up, pat it dry with paper towels (very important!), and brush it all over with the rest of the oil and herb mixture.
Roast the chicken for 30 minutes. Pour the wine into the pan (not on the chicken!) and roast for another 10 to 15 minutes, until a meat thermometer inserted into the thickest part of the breast registers 155 to 160 degrees.
Remove the chicken from the oven, sprinkle it with the lemon juice, cover the skillet tightly with aluminum foil, and allow to rest for 10 to 15 minutes. Cut the chicken into quarters or eighths, sprinkle with salt, and serve hot with the pan juices, cooked lemon, and onion.