so yummy! spinach gratin with white sauce(bechamel sauce)/yummy recipe/Figen Ararat

so yummy! spinach gratin with white sauce(béchamel sauce)/yummy recipe/Figen Ararat
spinach gratin with white sauce(bechamel sauce);
1 kg of spinach
2-3 tablespoons of olive oil
1 onion
1 teaspoon of salt
1 teaspoon of black pepper
for white sauce(bechamel sauce);
3 tablespoons of butter
3 tablespoons of flour
3 cups (600ml) milk
salt
black pepper
For the above;
100 gr cheddar cheese