In medium bowl, mix milk, cornstarch salt, dill and mustard until smooth; set aside.
In 8-inch nonstick skillet, melt butter over medium-high heat. Add mushrooms; cook about 2 minutes, stirring occasionally.
Reduce heat to medium. Gradually add cornstarch mixture, cooking and stirring until bubbly and thickened. Remove from heat; cover to keep warm.
Set oven control to broil. Top slices of bread evenly with ham; place on ungreased cookie sheet. Broil 4 to 6 inches from heat 2 to 3 minutes or until thoroughly heated. Serve mushroom mixture over ham-topped rye bread.