Melt the butter in a large, wide ovenproof pan over medium heat. Add the spring onions and garlic with a pinch of salt and pepper and cook, stirring, until completely soft and wilted — about 4–5 minutes. While this is happening, bring a large pot of well-salted water to a boil for the pasta.
sea salt, 3 tablespoons unsalted butter, 6 spring onions, 1 garlic clove
Sprinkle in the flour and stir to coat. Pour in the milk, stirring constantly, and simmer for 4–5 minutes until the sauce begins to thicken. Meanwhile, cook the shell pasta for 3 minutes less than the packet says, scoop out 2 cups (500 ml) of the starchy cooking water and set aside, then drain. Stir the reserved pasta water into the sauce and remove from the heat.
1 lb shell pasta, 2 tablespoons all-purpose flour, 2 cups whole milk, 2 cups reserved pasta cooking water
Crumble in the feta, add half the graviera (3½ oz / 100 g), the chilli flakes, oregano, and parsley. Stir well — the feta will melt into the sauce as you stir, creating a creamy, flavorful base. Taste for salt.
½ cup Greek feta cheese, ¾ cup graviera cheese, 1 teaspoon chili flakes, ½ cup fresh oregano leaves , ¼ bunch fresh flat-leaf parsley, sea salt, freshly ground black pepper
Preheat the oven to 350°F (180°C). Add the drained pasta to the pan and stir well to coat every shell in the sauce. Scatter the remaining graviera evenly over the top and more chili flakes if you like, and bake for 30 minutes until golden and bubbling. Rest for 5 minutes before serving.