Peel the potatoes with whatever is handy to you and while you are doing this, leave the peeled ones into a large bowl with cold water.
Then, remove the potatoes from the bowl and transfer them into a large pan, cutting each potato into two equal pieces.
Put the pan onto the cooker over a high heat and cover the potatoes with boiling water from the kettle.
Add a couple of teaspoons of salt and bring to boil (over high heat it should take a couple of minutes).
When the water starts boiling, switch the heat to low and wait for 7 minutes. You do not need to boil the potatoes too much because we are not going to mash them. While boiling the potatoes, pre-heat the oven to 190°C (375°F).
Drain the potatoes using a colander.
Leave the colander onto the table for few minutes so the potatoes dry up a bit and meanwhile go to the next step.
Pour the olive oil into the tin and swirl it around in order to cover the bottom of the tin.
Put the tin onto the top shelf of the oven and wait few minutes until the oil becomes hot (5 minutes is enough if the oven has been pre-heated previously).
Take the tin with the hot olive oil out of the oven and put the tin onto the cooker over low heat to maintain the temperature of the oil (at this stage an oven glove will be absolutely necessary!)
Start laying the potatoes into the tin (be cautious because the oil is very hot). If the potatoes are still a bit wet, this can cause some spurting when in contact with the hot oil, so using a long-handled spoon may be a good idea.
Once all your potatoes are into the tin, tilt the tin on one side so it is easier to collect the hot oil with a spoon and baste the potatoes.
Then, cut the rosemary sprigs in smaller pieces and spread these around with the garlic (unpeeled) and the sage leaves.
Now, with the oven set to 190°C (375°F), put the tin onto the top shelf of the oven and roast for about 40-50 minutes.
After 20-25 minutes, I suggest taking the tin out of the oven and turning all the potatoes onto the other side. Then, continue to roast for the remaining 20-25 minutes or until you think that the potatoes are at the right stage of crispness and goldness.
After 40-50 minutes you should have this result. Sprinkle with salt when the potatoes are still hot and then serve as side dish.