Add the rice, two teaspoons of oil, a quarter-teaspoon of salt and 400ml boiling water to a medium saucepan, for which you have a lid, on a medium-high heat. Turn the heat to low, cover the pan and leave to cook for 12 minutes. Remove from the heat and set aside, covered, for 10 minutes before fluffing up with a fork.
While the rice is cooking, heat three tablespoons of oil in a very small pan on a medium heat. Once hot, fry the sliced garlic, stirring to separate the slices, until pale golden – about two minutes. Watch that you don’t cook it further, or it may burn and turn bitter.
Use a slotted spoon to transfer the garlic to a paper-lined plate, reserving the oil. Sprinkle the fried garlic with a little salt and set aside.
Add the butter, remaining two tablespoons of oil, onions and crushed garlic to a large, nonstick saute pan on a high heat. Fry for about 14 minutes, resisting the urge to stir too often, until the onions are a deep golden-brown. The onions should catch on the bottom of the pan every now and then, but shouldn’t burn, so turn the heat down if necessary.
Reduce the heat to medium-high, add the chestnuts, black garlic, Cajun spices and a half-teaspoon of salt, and continue to fry for seven minutes, stirring every now and then, until everything is a deep, dark-brown colour, but not burned.
Stir the rice into the mixture until fully combined, and increase the heat to high. Once the rice at the bottom of the pan begins to crisp up a little – two to three minutes – add the stock and cook, undisturbed, until it has evaporated – another two minutes or so. Remove from the heat, stir through the parsley, garlic slices, lemon juice and reserved garlic oil, and serve.