In general, leeks can be substituted for onions in most dishes that call on onions for flavoring. The exception would be if the recipe calls for a very mild sweet onion or perhaps green onions. Keep in mind, substituting onions for leeks will result in a much stronger flavor.
Raw leeks can be sliced thin and added to salads, but I like to either roast or saute leeks with chicken stock, wine and garlic. What a heavenly smell! Leeks partner well with veal, cheese, chicken, and ham.
This recipe for Potato Leek Soup is my go to recipe when we have extra ham because it's so easy to make and it's such good comfort food. You can choose to add the extra cream or not. You can puree the soup for a smooth consistency, or just a few pulses for something more chunky and rustic like I do. Experiment as well with herbs or added cheeses.
You don't want the leeks to brown, they will taste burnt. Add 2 cloves of minced garlic and stir for 30 seconds. Then add 1/2 cup of white wine and bring to a boil. Okay so I added a wee bit more than a 1/2 cup.
Once the wine cooks down by about half, add your seasonings. I added 1/2 teaspoon ground white pepper, 1 teaspoon salt, just a pinch of nutmeg, 2 bay leaves, 1/8 teaspoon paprika, 1/2 teaspoon ground thyme (or a spring of fresh thyme), 1/2 teaspoon dried chives (or fresh chives).
Jim usually rolls his eyes when I serve soup for dinner and although he didn't think he would liked it, I won him over with his first bowl. I think your family will also warm up quickly to my potato leek soup with chopped ham.
For another take on soup with leeks, read my Roasted Potato Leek Soup recipe.