How to Make It
Heat the oven to 400°. In a 10- or 12-inch ovenproof frying pan or large saucepan, melt 3 tablespoons of the butter over moderate heat. Add the onion and carrots and cook, covered, stirring occasionally, until the onion is translucent, about 5 minutes. Add the mushrooms, 1/4 teaspoon of the salt, and the pepper. Cook, covered, stirring occasionally, for 5 minutes longer.
Stir the flour into the vegetables. Whisk in the broth, half-and-half, marjoram, and the remaining 1/2 teaspoon salt. Bring to a simmer and continue simmering until the sauce starts to thicken, about 2 minutes. Remove from the heat and add the chicken. If using a saucepan, pour the mixture into an 8-by-12-inch baking dish.
In a medium saucepan, melt the remaining 2 tablespoons butter. Remove from the heat and stir in the bread crumbs. Sprinkle the buttered bread crumbs in an even layer over the chicken mixture. Bake in the oven until the crumbs begin to brown, 10 to 12 minutes.