Cut the lemons in half and squeeze them into a zipper bag large enough to hold all of the ingredients. Don't worry if there are pips, they won't make any difference to the finished dish.
Add the onions, rosemary, chicken and olive oil. Close the bag and give it a good old massage to mix all the ingredients together and start the marinating process.
Leave the bag of chicken and marinade in the fridge for as long as you can wait - 2 days is ideal.
When you're ready to cook, take the chicken out of the fridge to get it to room temperature and pre-heat the oven to gas mark 7, 220°C (200°C fan). Line a baking tray with foil - if you don't want to spend the next two days cleaning the baking tray, this is essential!
Empty the bag of chicken and the rest of the marinade into the baking tray and then arrange the chicken to be on top of the onions and lemons.
Mix the sambal oelek with a little olive oil just to loosen it up a little and then brush all over the chicken getting into the crevices.
Cook the chicken for 45 minutes and then turn the oven down to gas mark 2, 150°C (140°C fan) and you can leave it for as long as you like - it won't dry out. Serve with our caesar salad.