In a large pot, melt the butter, sprinkle the flour and mix to form a paste.
Add the milk, veggie broth or water and mustard. Mix until no more lumps.
Once simmering, add the garlic, carrots and potatoes.
Cook for 8-10 minutes, stirring occasionally.
Add the peas and spinach, season with salt and pepper, and cook for another 5 minutes.
Take off heat, add the green onion and parsley, mix well.
Transfer the mixture to the baking dish and cover with pastry dough.
Cook for 15-20 minutes at 180 degrees C.