In a large skillet, sauté the onion in the remaining butter until tender. Stir in the flour, cooking for 2 to 3 minutes. Combine the mushroom juice with enough broth to make 3 cups of liquid. Slowly stir the juice-broth mixture into the onion mixture. Stir in the half-and-half. Cook until the mixture is thickened. Add the sautéed or canned mushrooms, the chicken, rice, toasted almonds, pimiento, parsley, salt, and pepper. Pour into the prepared casserole dish. Bake, uncovered, for 30 to 45 minutes, until most of the liquid is absorbed.