Place the peeled potatoes into a large pot of cold water. Bring to the boil. Once boiling, add the 2 teaspoons salt. Cook until the edges have softened but the centres remain firm, about 10-15 minutes.
Drain into a colander and leave to steam dry for at least 30 minutes. Let them dry as long as possible for best results.
Preheat oven to 220℃ (200℃ fan forced). Line a deep baking tray with baking paper. Add the 1/4 cup oil and heat in the oven for 5 to 10 minutes.
Return the dried potatoes to the saucepan. Add 1 tablespoon oil and the semolina. Shake or toss well to rough up the edges.
Carefully remove the hot tray from the oven. Add the potatoes to the hot oil and turn to coat. Place all potatoes flat side down.
Bake for 40-45 minutes. Check the bases are deep golden and crisp. Turn and bake a further 15 to 20 minutes until evenly golden.
Transfer to a serving bowl. Sprinkle with sea salt flakes and rosemary if using. Serve immediately.