2) Add half a teaspoon of extra-virgin olive oil, half a turn of the pan, and the onions. Cook for five minutes to soften, then put them in mixing bowl to cool.
3) While onions cool, put the milk, feta, half of the marjoram or oregano, mint, salt and pepper, to taste, and the nuts in a food processor and process to a thick, smooth sauce.
4) Add lamb to the bowl with the onions and season with salt and pepper, to taste. Stir in the remaining marjoram or oregano, paprika and cinnamon. Form the mixture into four patties, thinner at the center and thicker at the edges. Heat half a tablespoon of extra-virgin olive oil, in nonstick frying-pan over medium-high heat and cook the patties three to four minutes on each side for pink centres.
5) Meanwhile, heat two tablespoons of extra-virgin olive oil in another frying-pan over medium-low heat. Add the garlic clove and let it infuse the oil for one to two minutes. Add the spinach and let it wilt, then remove the pan from the heat, and season with salt, pepper and nutmeg, to taste.
6) Add the lamb patties to the bap bottoms or charred bread. Shake off excess liquid as you remove the spinach and pile it on top of the burgers. Slather the bun tops with the feta sauce. Cover the burgers with the bap tops and serve.