Hot Open-Faced Ham, Swiss Cheese, and Mushroom Sandwiches

In a small bowl whisk together the mayonnaise, the mustard, the dill, and salt and pepper to taste. In a skillet cook the mushrooms in the oil over moderately high heat, stirring occasionally, for 5 minutes, or until the liquid the mushrooms give off is evaporated, and remove the skillet from the heat. On a baking sheet spread one side of each slice of toast with half the mayonnaise mixture, top each toast with a slice of the ham, half the mushrooms, a slice of the Swiss cheese, half the onion, and salt and pepper to taste, and broil the sandwiches under a preheated broiler about 4 inches from the heat for 1 to 2 minutes, or until the cheese is melted and browned lightly.