Salt chicken inside and out; tie legs and trim wing tips. Refrigerate several hours or overnight uncovered or at least 1 hour on counter.
Place a cast-iron skillet in the oven and preheat oven to 425°F.
Toss fennel, potatoes, onions, half the thyme, some salt and pepper in a bowl with about 3 tablespoons EVOO. Pat chicken dry then drizzle with oil and season with remaining thyme, black pepper and fennel seed.
Drizzle the garlic with oil and season with salt and pepper.
Remove the hot skillet from the oven and add vegetables. Nest garlic in the veggies cut-side up then set chicken on top. Roast 1 hour, adding lemon halves to the pan in the last 20 minutes of cooking time. Chicken should have an internal temperature of 160°F at the thigh.
Set chicken on a cutting board to rest for 15-20 minutes. Remove veggies with a slotted spoon to a bowl or plate and reserve. Press garlic from their skins and paste it in a small bowl.
Place the skillet with the pan drippings over medium heat and add white wine. Whisk in pasted garlic and stock, and bring to bubble. Remove pan from heat and add cream and juice of 1/2 lemon.
Carve chicken and douse meat with juice of remaining lemon half.
Serve chicken with fennel and potatoes alongside, with some roasted garlic cream spooned over everything.