My top two suggestions: Potato Leek Soup, and Prasopita.
There is much debate about if you should use the dark green tops or only the white part. I say for soups, definitely use the dark green parts that look good. It adds more dimension to the flavor. For other uses, you probably do not want to. I've worked in places that would save most of the tops for soups (with some whites) and use the whites for other dishes. But, it's all about what you like once you figure that out.
A very basic (and cheap) soup of just onion (optional?), celery (optional?), thyme, leeks, potatoes, parsley, salt and pepper is hard to screw up. I use Olive Oil, but that's because I keep the recipe vegan out habit for the restaurant I made a lot of soups in, some people use butter. I honestly prefer Olive Oil (not EVOO), because the butter (or EVOO) stands out too much IMO. Never burn/brown leeks, and for this always have a bit more leek than potato. Somewhere between 4:3 and 2:1 ratio of leek to potato, but closer to 4:3 is good. You can just eyeball this and it doesn't have to be exact. You'll figure out what you prefer after a few experiments. Just use all of your leek, and base the potato on that. If you put in too much potato, it ruins the soup, trust me on this. I just throw the celery (2 ribs), onions and thyme in and sweat them with a lid on and stir occasionally. Sweat for 5 mins. Stir. Add leeks and some salt. Sweat for like 10, stirring a few times. Let them release water and cook down some. Use a lower heat. Low and slow preserves flavor. Keep a lid on. It preserves flavor and sweats better. Add potatoes and just enough water to cover (you can always just reduce it down if you add too much, but try to not do this, it is easier to add more later). Boil until potatoes and celery are soft. Add fresh chopped parsley. Puree. salt and pepper to taste. Some people add cream, use chicken stock, don't use onions or celery etc etc, but I have had the most success with this to please other people. I myself recommend NOT using stock or cream. It won't need it. Good fresh ingredients that go well together speak for themselves. I've had versions with stock, cream and other stuff. Simple is good. Something too busy drowns out the leeks. The stock can take over the flavor, and the richness from the cream is okay, but totally frivolous IMO. Some people use dill. I don't know about that. You may want to experiment with what potato you prefer. Flavors and textures change, everyone will have their own preference. Personally, I prefer russet for this soup, but that is because they let the leek stand out more. Normally I suggest golden potatoes for soups though. Also, taste the potatoes when baked that you will use before making this. If the potatoes do not taste good (ie too earthy), it will ruin this soup. Now, I will say that you can also swap out or mix up other roots at the same ratio with good results. Carrot-leek, turnip-leek, parsnip-leek, a mix of them all at this same ratio, all come out pretty good. I made a smokey bacon, san marzano leek soup that was great too. Roasted cauliflower and leek or sauteed broccolli rabe and leek is my next experiment.
Prasopita is a delicious Greek pastry. I make it with just a phyllo crust, sauteed leeks stems (white part), eggs, goat cheese, black pepper, and parsley. This is not a standard recipe, but it is delicious. Supposedy some other cheese is used, But I use goat cheese because it s easier to find, and I can get away with not using milk which many recipes call for (I am lactarded, and have never liked milk). You can slo use a lot less salt. I've had Greeks commend me on my prasopita, so I may be onto something. I'm sure it varies and each village changes it up anyways. Some people use mint and dill and lemon zest and other stuff. Do so at your own risk. Simple is good with something as delicious as leeks. Basically, saute the leeks, cool, mix with the all the other ingredients besides the phyllo to make a filling, and the rest is obvious, or on the google. Since it is baking, you may want to find a recipe.
There is a lot more you can do with leeks, but I find just about anyone enjoys these dishes. You can use them in place of onions, but not vice-versa in any case I know of. They're way more delicious. Remember, do not brown them. They do not caramalize deliciously like onions do. They get nasty and bitter. They're good in pot-pies, some people put them in Borscht, but that soup is an institution, and doing so is offensive to many Ukrainians and Russians. Hope this helps or gets you to enjoy them. I think they are underrated and delicious.