Preheat the oven to 220ºc.
Trim off the tough root end and any stalks or fronds from the fennel. Cut in half lengthwise, and slice into 1cm thick wedges.
Pat the chicken highs dry with paper towels. Season with 2 1/2 teaspoons of salt and a few grinds of coarse black pepper. Arrange the thighs skin-down in a dry and unheated 30cm cast-iron pan. Set over a medium heat and cook, undisturbed, util the skin releases easily from the pan and is light golden brown - 12 to 14 minutes.
Once released, move the chicken thighs around a bit to ensure even browning. Continue to cook until the skin is very crispy and deep golden brown, 4 to 6 minutes longer. Use tongues to transfer the chicken to a plate skin-side up.
Using tongs, add the fennel wedges to the pan, turning them to coat in some of the rendered chicken fat. Season with 1/2 tsp salt. Return the chicken to the pan skin-side up. Transfer the pan to the oven and roast 10 to 15 minutes, flipping the fennel halfway through for even browning, until the chicken meat is no longer pink and the fennel is caramelised.
Meanwhile, thinly slice 6 spring onions and 3 garlic cloves.
When the chicken is ready, remove the pan from the oven and transfer the thighs to a plate, skin-side up to stay nice and crisp.
Add 50g butter, the spring onions and the garlic to the fennel in the pan. Cook on a medium heat until the spring onions and garlic have softened, but not browned, 1 to 2 minutes.
Stir in 250g peas, 1/2 tsp chilli flakes, 125mil water and 1/2 tsp salt, scraping up any browned bits from the bottom of the pan, and cook until the peas are bright green and just cooked through, 3 to 4 minutes. Turn off the heat.
Stir in the juice of 1/2 lemon. Taste the peas and add more salt or lemon juice as needed until it tastes super lemony and really tasty.