My father used to make this traditional Belgian dish for my family in winter. Feel free to use white fish or chicken (for this recipe I've used chicken) -- both are delicious. It's the ideal comfort food, with loads of vegetables, and can be eaten with potatoes or thick Belgian bread. —MarieGlobetrotter
Peel and wash the leeks, then chop them into thin slices. Dice the carrots and celery.
Cut the chicken breasts into medium slices, about 1/2-inch thick.
In a deep pot, melt the butter and sauté the vegetables and the onion for 10 minutes. Add enough chicken broth to just cover the vegetables and bring to a boil, uncovered.
Add the chicken breast and cover the pot. Poach the chicken on medium heat for approximately 20 minutes, or until the chicken is cooked through. Remove the chicken and set aside. Drape some aluminum foil over top to keep it warm.
In a bowl, whisk the cream, cornstarch, and egg yolk. Add this mixture to the broth and vegetables. Add more broth to thin the mixture as desired. Stir constantly until the waterzooi thickens.
Add the chicken, season with salt and pepper, and cook for 2 to 3 more minutes at a low simmer.