Poached Halibut with Vegetables and Mustard Sauce

About 2 cups any fish, chicken, or mild vegetable stock
3 tbsp butter
2 medium carrots, peeled and diced
2 medium onions, diced
2 stalks celery, diced
1 clove garlic, minced
Salt and freshly ground black pepper to taste
2 halibut steaks, 3/4 to 1 pound each

1 tbsp butter
1 tbsp flour
1 to 2 tbsp Dijon mustard
minced parsley for garnish

In a small saucepan, bring the stock to a boil. Lower the heat, and keep it warm.

In the smallest skillet or casserole that can hold the fish steaks, melt the butter over medium heat. Add all ingredients except fish and stock and cook, stirring, until the vegetables wilt, 5 to 10 minutes.

Place the steaks on top of the vegetables and add the stock. Simmer over medium-low heat until the halibut is almost done, about 8-10 minutes (a thin-bladed knife inserted between bone and flesh should reveal little or no translucence). Remove the steaks and vegetables from the pan, and keep them all warm in a warm oven.

Melt 1 tbsp butter in a small saucepan over medium-low heat. Add the flour and cook, stirring, until the mixture turns nut-brown, about 3 or 4 minutes. Strain 1 cup of the fish-cooking liquid and whisk it gradually into the butter-flour mixture, stirring constantly to eliminate lumps. Cook until slightly thickened, 4 or 5 minutes. Season with salt and pepper and add 1 to 2 tbsp Dijon mustard, or to taste. Serve the halibut and vegetables topped with the sauce and minced parsley.

-Mark Bittman, How to Cook Everything