Spinach Mushroom Casserole #food

Spinach mushroom casserole with potatoes, one of my favorite comfort dish for winter.
Here is the full video with more details and the recipe: https://youtu.be/_kojdSmExJM
Ingredients For Spinach Mushroom Casserole With Meat and Mashed Potato
For the potato pur:
5 big potatoes
3-4 tbsp butter
1 cup or more whole milk
salt to taste
For the Spinach Mushroom Layer:
200-300gr. cubed beef or chicken (optional)
1 onion
4 tbsp olive oil
300gr. spinach
200gr. mushrooms
salt, black pepper to taste
For the Bechamel Sauce:
3 tbsp olive oil or 2 tbsp butter
2 heaped tbsp all-purpose flour
1+1/2 cup milk
dash of nutmeg
salt to taste
grated mozzarella or fresh kashar cheese for topping