For the marinade, in a small mixing bowl, combine 4 tbsp of the oil, the lemon and tangerine juices, the wine, mustard, Grand Marnier, brown sugar, paprika, red pepper flakes, onion, 2 of the thyme sprigs, 1½ teaspoons of salt, and ½ teaspoon of pepper.
Place the chicken in a large plastic bag.
Pour in the marinade, seal, and turn to coat completely.
Marinate in the refrigerator for at least 1 hour.
Remove the chicken from the refrigerator, ideally 1 hour before cooking if you have marinated it overnight.
Set the racks in the middle and upper third of the oven.
Preheat the oven to 230ºC.
Place a colander over a large mixing bowl and drain the chicken, reserving the marinade along with the onion and thyme.
Remove the chicken and dry thoroughly with paper towels.
Pour the marinade, onion and thyme into a rimmed baking sheet.
In a large pan, heat 1 tablespoon of the remaining oil over medium-high heat until shimmering.
Add half the chicken pieces skin-side-down and reduce the heat to medium.
Cook for 4-5 minutes, undisturbed, until a dark golden crust forms.
Remove the chicken from the skillet and place on the baking sheet, skinside-up, on top of the marinade.
Wipe the pan clean.
Add the remaining tablespoon of oil and repeat with the remaining chicken.
Transfer to the baking sheet, reserving the oil in the pan.
Turn off the heat and let the pan cool for 1 minute.
Add the garlic to the oil in the pan and turn the heat to medium.
Cook the garlic for 3 minutes, flipping it
halfway through until lightly browned on both sides.
Transfer to the baking sheet with the chicken.
Arrange the lemon and tangerine slices around and under the chicken.
Lay three sprigs of thyme on top and season.
Bake in the middle of the oven for 25-30 minutes, until the chicken juices run clear.
Remove the baking tray from the oven and move the chicken to a serving plate.
Return the baking tray with the garlic and fruit to the oven.
Cook for 4-5 minutes until the citrus slices caramelise.
Remove from the oven and arrange the fruit, garlic and onion around and on the chicken.
Pour the liquid from the baking tray into a small saucepan and bring to the boil.
Reduce the heat and simmer for 8-10 minutes, until reduced by a third.
Strain the sauce and serve separately.