A little twist on this soup is to first oven roast the leeks and potatoes. It gives the finished product a greater depth of flavor. Yet the process is simple, it is a bit more time consuming.
First you cut and clean the white part of the leek and place on parchment paper in a shallow baking pan. Cut and quarter potatoes and place on parchment paper in a separate baking pan. Drizzle both with olive oil and liberally salt and pepper them. Roast at 350 degrees.
The leeks (I used three) cook faster. So watch for doneness, a little charred on the edges and pull out when finish. Let the potatoes continue to cook until fork tender. Leeks took about 20 minutes. Potatoes took 45 minutes. This is why they are in separate baking dishes.
When everything is cooked, proceed to make the soup. Saute one half of an onion finely diced and one quarter cup of celery chopped in a knob of butter for a few minutes. Add two minced garlic cloves, a one teaspoon each of dry mustard, ground cumin, and ground coriander. Add roasted vegetables and box chicken stock to cover. Simmer.
When the soup is grooving, turn off the heat and let sit. Get out your blender or immersion blender. I used a regular blender. In batches, liquefy. Return to pot and add milk to get a looser consistency. I suppose you could use stock or water if you wanted. I want the calcium, so milk is the ticket.
Turn on the stove top to heat (do not boil or heat too fast) and adjust seasoning if necessary. Served here with roasted chicken and sliced bread.