As a retired chef (in the States) this is a topic that has interested me since living here (Siem Reap) the last 5 years.
In general, Khmer cuisine is not as "sophisticated" as it's larger neighbors, Thailand and Vietnam. For instance the Chili Pastes (Kroeungs) have fewer ingredients than Thai curry pastes and the Vietnamese have incorporated more French influences in their cuisine than the Khmer people have.
If you are really talking LOCAL, you'd be cooking with wood or charcoal, depending on what you were cooking. I live on a compound with two Khmer family entities plus my partner and me and we (the latter) are the only ones with a gas range w/ an electric oven. The others still cook traditionally.
Ingredients I would say are typical of many Khmer dishes are Fresh Ginger, Galangal, Lemongrass, Kafir Lime leaves, "Bird"Chilies [small, very hot], shallots (small), Dried Shrimp, Garlic + "chee" (herbs) like Coriander, Saw Mint, Spring Onion, Thai Basil, Holy (Khmer) Basil and Rice Paddy Herb (Máam) to name only a few. Black Pepper is also commonly used and some of the finest Black Pepper in the world is grown in Kampot, Cambodia. One surprise I had when I joined my Khmer family was that dried Black Peppercorns are not used "raw" here as they are in the West. They are roasted before they are crushed or put in a pepper grinder........they are actually better than those I'd been using in the West. Another aspect of Khmer cuisine that I have experienced is that soups are served with rice while "stews" (which tend to be sweet!) are frequently served with a baguette (a nod to the French). Soups and Salads are predominant in Khmer cuisine. Finally, a word has to be said about Prahok, a fermented fish paste that is salty and strong fishy tasting that is used in dips and a flavoring agent. You can buy Prahok in the market but if you aren't sure what variety of fish was used to make the Prahok, it needs to be thoroughly heated, before using, to be sure there are no parasites in the paste........It is wonderful and a little goes a long way. A simple, yet great, sauce served with street food is just salt with red chilis and lime juice.
As far as cooking classes are concerned, I have seen ads for such classes around Siem Reap (and some in Phnom Penh when visiting), but I don't know how good they are. I use a couple of books that are pretty helpful. One is an old book that I still see at book stands, "The Book of Khmer Cooking" - hardcover, pinkish w/ photo of Amok, which is VERY poorly edited........for instance make sure you read the "Method" section because it might talk about ingredients that were NOT listed in the "Ingredients" section. It is very quaint in it's descriptions, like "let sit on flame till bubble with aroma" means boil it until it smells good.......stuff like that. It's a really fun book. Another book I like is modern and has a lot of good details on ingredients and cooking methods. It is titled, "Authentic Cambodian Recipes [from mother to daughter]" by Sorey Long. My only problem with this book is that they didn't include more recipes [like 30 more!!!].......probably a matter of publishing cost because of so many photos.....too bad for us.
Easily prepared dishes you MUST try: Burnt Chicken [Khmer free range] w/ Watergrass, Pig Leg Soup w/ Preserved Radish, Caramelized Pig Trotters [use Palm Sugar!] and SIMPLE Chicken Sour Soup w/ Herbs.......YUM, YUM, YUM and YUM!!!!