Wash the chicken legs and pat dry. Season all over with salt and pepper.
Squeeze 4 lemons to make roughly 1/8 liter of juice. Mix this juice with 1/8 liter of olive oil and about 1 teaspoon of salt. Stir until the mixture has a creamy consistency.
Place the chicken legs in a baking dish so they fit snugly. Pour the marinade over them. Add 3 sprigs of rosemary and all 10 sage leaves. Cover and refrigerate for at least 3 hours (you can marinate up to 2 days, turning the chicken occasionally).
Preheat the oven to 250°C (482°F).
Peel the potatoes and cut them into 1 to 1.5 cm cubes. Heat 4 tablespoons of olive oil in a large pan over medium heat. Add the potatoes and fry covered for 15 minutes, stirring every 5 minutes.
While the potatoes are cooking, finely chop the leaves from 1 sprig of rosemary, the leaves from the thyme sprigs, and 1 garlic clove. Remove the pan from heat. Season the potatoes with salt, pepper, and 2 pinches of chili powder. Mix in the chopped rosemary, thyme, and garlic. Set aside.
Halve or quarter the cherry tomatoes depending on their size. Halve the dried black olives and remove the seeds.
Transfer the chicken legs and marinade to a baking sheet. Add the rosemary sprigs and sage leaves from the marinade. Cut the 4 garlic bulbs in half lengthwise and arrange them between the chicken legs. Add the tomatoes and olives. Distribute the fried potatoes among the other ingredients. Press the olives partially under the other ingredients so they don't brown too quickly.
Bake for 20 minutes at 250°C (482°F). Reduce the heat to 220°C (428°F) with the fan (convection) on. Place 2 lemon slices on each chicken leg and bake for 25 to 30 minutes more until the chicken reaches 74°C (165°F) in the thickest part.
Remove the baking sheet from the oven and sprinkle everything with the finely chopped basil leaves. Serve immediately.
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