Put whole peeled potatoes in a medium-sized pan. Pour enough cold water to submerge the potatoes. Add 1 tsp salt. Bring to a boil and let simmer for 10 to 15 minutes, until the potatoes are a bit soft (BUT not completely cooked since you still need to put the potatoes in the oven).
Remove potatoes from the pan and set aside to cool.
Preheat oven to 220-degrees C (450-degrees F).
Slice potatoes into cubes. Try to keep the slices in the same size, this helps in cooking them at the same time (and avoiding burning small bits).
In a bowl, combine potatoes, 1 tbsp olive oil, ½ tsp of salt, ¼ tsp of pepper and mix to ensure that each slice of potato is coated. Transfer potatoes in a baking sheet. Make sure that the potatoes are not touching each other.
Cook in the oven for 12 to 15 minutes. Turn the potatoes and cook again for another 10 minutes.
While you are roasting the potatoes, you can make the sauce. In a small pan, pour 2 tsp of olive oil and once ready, cook garlic and onion until soft. Add tomato sauce and the spices. Cover and bring to a boil. Taste and adjust seasoning (including salt and pepper) if desired. Lower the heat and simmer for 10 minutes. Set aside to cool.
Put the cooled sauce in a blender and blitz. If the sauce is a bit too grainy or clumpy, use a strainer to get a smoother sauce. This is now ready for serving.
Take the potatoes out of the oven and pour a bit of the sauce on top of the potatoes to serve.