Put a Dutch oven on medium high heat and pour the olive oil in it.
When the olive oil is hot, put the pork tenderloin in the Dutch oven and cook it for about 2 minutes on both sides until it gets a golden color.
Pour the orange juice in the Dutch oven. As explained above, the quantity of orange juice will vary according to the size of your pork tenderloin and the size of your Dutch oven.
Then let it cook on medium high heat (the orange juice should be on the verge of boiling but not boil completely) for about 20-25 minutes. At half the time, flip the pork tenderloin on the other side.
In the meantime, using a knife, peel the oranges raw and slice them. Add the orange slices to the Dutch oven at the end of the cooking process (for about 5 minutes only).
To make sure that the pork tenderloin is cooked, slice it in the middle and look at its colour: it should NOT be pink.
Once the meat is cooked remove it along with the orange slices from the Dutch oven (leave the orange juice in the Dutch oven). And set them on a plate.
Take a seperate bowl and transfer the arrowroot in it. Using a ladle or a very large spoon take about ½ cup (125ml) of orange juice out of the Dutch oven and pour it slowly over the arrowroot while whisking. Once the arrowroot is well combined to the orange juice, transfer the bowl content to the Dutch oven and mix the 2 juices together (I find that this method avoids getting lumps rather than if you pour the arrowroot directly in the Dutch oven).
Pour part of the orange juice over the meat and put the rest in a sauce boat. Et voilà! Serve immediately with cauliflower rice (if you are on AIP, paleo or whole30) or serve with rice (if you are not on any of those diets).Bon appétit!