Preheat your oven to 450°F. Arrange the sliced onion, garlic, and lemon slices in a single layer in a large (12-inch) cast-iron skillet.

In a small food processor or a spice grinder, combine the thyme leaves, fennel seeds, kosher salt, and black pepper. Pulse until the fennel seeds are coarsely ground. Transfer to a small bowl, add the olive oil, and whisk to combine.

Place the butterflied chicken on a cutting board and pat it very dry on both sides with paper towels—this is crucial for crispy skin.

Brush about one-third of the herb-oil paste onto the underside of the chicken. Set the chicken, breast side up, on top of the onion mixture in the skillet. Pat the top and sides dry again with paper towels, then brush the remaining herb-oil paste all over the top and sides, getting into all the nooks and crannies.

Nestle the lemon halves, cut side up, into the skillet around the chicken.

Roast the chicken for 35 minutes. Remove the skillet from the oven and carefully pour the white wine around the chicken (avoid pouring it directly on the skin). Return the skillet to the oven and continue roasting for another 15 to 20 minutes, until an instant-read thermometer inserted into the thickest part of the breast (not touching bone) registers 160°F to 165°F.

Remove the skillet from the oven. Squeeze the juice from the roasted lemon halves all over the chicken and the pan juices. For the crispiest skin, do not cover; let the chicken rest uncovered for 10 to 15 minutes. If you prefer a softer skin, tent loosely with foil.

Carve the chicken into serving pieces (the spatchcocking makes this easy). Sprinkle with flaky sea salt if desired, and serve hot with all the pan juices spooned over the top.