Put the oil into a small saucepan over medium-high heat. Toss in the oregano, bay, porcini, pimento and mace. As soon as it starts to simmer, turn the heat down to low. Gently infuse the oil with the herbs and spices, without boiling, for 20 minutes.
You don’t want to burn the herbs and spices, so if they turn dark brown before the 20 minutes is done, it’s time turn the heat off.
Allow the saucepan to sit, off the heat, to cool for 10 minutes.
Put the chilli flakes and salt into a heatproof bowl. Lay a fine mesh sieve over the bowl. Pour the hot oil through the strainer to catch the herbs and spices. It may start to spit or bubble, so be careful.
Drain and discard the herbs and spices in the strainer, give the oil and chilli a good stir and let it cool completely before storing in a jar in the pantry.
If you wish, you can strain off the chilli flakes, discard them, and keep the oil nice and clear.