Begin by meliting the butter in a pot big enough to fit all the ingredients mixed together. Add diced leeks and sliced mushrooms until they begin to soften (about 10-12 minutes). Lightly sprinkle in the flour a teaspoon at a time and stir with a wooden spoon. Add chicken stock and stir again. Set in the chicken thighs in a circle in the middle of the pot and push the leeks and mushrooms around it to the outer edge. Sprinkle in some more of the flour. As the fat cooks off the thighs, it will make it's own gravy. Turn the thighs as they brown on the bottoms to ensure thorough cooking.
I call this a one pot meal but I will add that I like to put the thighs and mixture over couscous so that would be cooked in another pot.
Served with a side of steamed spinach or broccoli.