Meanwhile, finely slice one bunch of spring onions, then cut the other bunch in half to roughly separate the white parts from the green. Heat 3 tbsp oil in a small frying pan over a medium heat and cook the finely sliced spring onions, the garlic, chilli and nigella seeds until the garlic is fragrant and sizzling, about 2-3 mins. Strain the mixture in a fine sieve set over a bowl, reserving the oil. Tip the yogurt and 150g feta into a food processor and blitz until smooth, or do this using a hand blender. Stir in half the spring onion and garlic mixture, along with some seasoning, and chill until needed. Will keep chilled for a day.