Preheat oven to 210c/410f.
Bring a large pot of water to boil, then add bicarb soda - it will fizz a little.
While the water is coming to the boil, peel your potatoes.
Once boiling, add the bicarb soda - it will fizz a little. Then carefully add the potatoes, and boil for 20 mins or until very soft.
When the potatoes have about 10 minutes to go, pour the oil into a metal roasting tin, and place in the oven to heat.
When potatoes are soft, drain well then return to the pot. Give them a mix around in the pot to rough them up, if they break apart a bit that's fine too.
Carefully add potatoes to now-heated oil, and turn well to coat them completely.
Bake for 1 - 1 1/4 hours, tossing again about every 15 mins (add rosemary and garlic cloves with approx 15 - 30 minutes to go if using).
Discard rosemary and garlic, add salt to taste and serve immediately.