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Preheat the oven to 200°C/Gas 6. Season the inside of the chicken. Place the tarragon and butter in a bowl and beat until combined. Season with salt and pepper. Loosen the skin over both chicken breasts by gently pushing your fingers underneath it. Now push the tarragon butter under the loosened skin so that it covers the whole crown.
To make the stuffing, put the chickpeas into a bowl, season and add the chillies, lemon zest, thyme leaves and a dash of olive oil. Mix well. Spoon the chickpea mixture inside the chicken cavity and place the whole lemon at the entrance.
Place the garlic heads, cut side down, in a roasting tin. Put the chicken on top and drizzle with olive oil. Season the outside of the chicken with salt and pepper and roast for 10-15 minutes, until turning golden and beginning to crisp up. Reduce the heat to 180°C/Gas 4 and continue roasting for 1¼-1½ hours, until cooked through and golden all over.
Extract the lemon from the cavity of the bird and spoon the stuffing into a large bowl. Place the chicken on a warm platter, cover loosely with foil and set aside to rest for 10-15 minutes.
Meanwhile, separate all the lettuce leaves and tear any really large ones into pieces. Place in a salad bowl along with the spinach and avocado.
Put all the dressing ingredients except the oil into a bowl and whisk well. Add a pinch of salt and pepper, then add the oil and mix until emulsified. Set aside.
Spoon the garlic out of the pan and squeeze the pulp into a sieve placed over the bowl of stuffing. Slice the roasted lemon in half and squeeze the juice over the garlic. Push the garlic and juice through the sieve with the back of a spoon. Mash the entire contents of the bowl with a potato masher, then add 1 tablespoon of the dressing. Mix well, then transfer to a serving bowl and drizzle with a little extra olive oil.
Add 2 tablespoons of the dressing to the salad, then toss, taste and adjust the seasoning as necessary, adding a little more dressing if needed. (Any leftover dressing can be stored in a screwtop jar in the fridge.) Serve immediately alongside the roast chicken and stuffing.