Ideas, Advice and Suggestions

Leeks, washed well and drained, making sure all grit and sand is removed: The leeks I usually buy are relatively clean, yet I still wash them thoroughly to avoid any grit in the final dish. When leeks are priced reasonably, I purchase them, clean, chop, and freeze them in 250g portions. This preparation saves time for recipes that are otherwise quite involved. The recipe recommends cooking the leeks for 12 to 14 minutes. However, if you're using frozen leeks or if they're a bit tough, the cooking time extends to about 20-25 minutes. Ensure they are completely tender; you should be able to squash them with a wooden spoon with almost no resistance.
Olive Oil: Any mild oils will work
Salt, divided: Add half of the mixture to the leeks and the other half to the milk. If you're a fan of salt, you might want to add more, but in my opinion, it doesn't need it. Consider adding a seasoning salt (be sure to check the label for gluten) or Zeal (MSG) to enhance the flavor.
Large eggs: Any size egg will work, but if you have particularly small eggs, consider adding an extra one.
Whole Milk (you could use 2%--it'll just be less creamy) : I enjoy fresh milk but have used UHT in the past; I haven't tried it with non-dairy alternatives yet. 
Corn-starch: If you're out of cornstarch and don't require a gluten-free option, wheat flour or any other starch can be used as a substitute.
White Pepper: Either white or slightly less black pepper can be used.
Cheese - Cheddar and Mature Cheddar: I enjoy experimenting with different cheeses and often have a variety on hand. Feel free to use any cheese you have available, but I would advise against using wet cheeses like fresh mozzarella or crumbly ones such as Feta or Halloumi, as they can alter the texture and add too much saltiness to the quiche.
Note Regarding Pepper: All pepper is Gluten free but Pepper Compounds are pepper mixed with flour is most cases.