Fırst of all, set your oven to preheat to
200 °C. Meanwhile, over a medium heat, gently heat your olive oil in your large frying pan and add your chopped onion.
After a few minutes, once your onion begins to soften, add your butter and allow to melt.
Now start to load your chopped spinach leaves into the pan. Keep stirring them so that they begin to wilt and you can fit the rest of them in the pan.
Stir them around for a few minutes and then cover the pan and reduce the heat to low so that your spinach leaves and onion and cook.
Once you are happy that your spinach leaves are wilted and cooked - after around 5 minutes - remove the lid and add your seasoning, paprika and garlic powder and stir them in.
Now sprinkle your flour into the pan and stir it in so that it soaks up your butter and oil.
Increase the heat to medium and add your milk a splash at a time. Keep stirring so that your white sauce remains smooth and thickens as you go.
Keep adding the milk, stirring as you go, until you are happy with the consistency.
Now add your grated parmesan and stir that in before pouring your spinach mixture into your shallow casserole dish.
If you are serving immediately, top the spinach mixture with your grated gruyere (gravyer peynir) and place in the oven to bake until bubbling and your cheese topping has melted (around 15 minutes).
If you are making your spinach gratin recipe ahead, leave your spinach mixture to cool and store, covered, in the fridge for up to three days. When you are ready to eat it, top with grated cheese and bake in the oven at
200 °C for 20-25 minutes until bubbling. Sprinkle hot chilli flakes over the top of your spinach gratin before serving, if you like.