Pre-heat the oven to gas mark 8, 230°C. Put the frozen potato slices in a bowl and toss them around in 1 teaspoon of the olive oil, then spread them out in a single layer on a baking sheet and pop them in the oven for 10 minutes. Meanwhile, toss the onion, garlic and rosemary in the other teaspoon of oil in the same bowl, then after the 10 minutes is up, sprinkle this mixture over the potatoes (holding the baking sheet with a cloth as you do this). Then slide it back into the oven for a further 20 minutes. Serve the potatoes sprinkled with rosemary flaked sea salt and freshly milled black pepper.