Peel and finely chop the onions and garlic, then place in a large frying pan on a low heat with 2 tablespoons of oil and the oregano.
Finely grate in half the nutmeg and fry gently for 10 minutes, or until soft but not coloured, stirring regularly.
Add the spinach, turn the heat up to medium and cook down for 15 to 20 minutes, or until any liquid has evaporated.
Meanwhile, roughly chop the cold butter, grate the cheese and place in a food processor with the flour, oats and a pinch of sea salt and black pepper. Pulse into a nice crumble texture (or rub together by hand), then remove to a plate.
Put the cooked spinach mixture into the processor (there’s no need to clean it), then add the crème fraîche and blitz for 1 minute.
Taste and season to perfection, then tip evenly into a baking dish (20cm x 30cm). Sprinkle over the crumble and bake for around 45 minutes, or until golden and bubbling.