Combine water, milk, and salt in a large saucepan. Bring to boiling. Reduce heat and slowly add grits, stirring with a whisk. Cook and stir for 5 to 7 minutes or until very thick. Remove from heat. Add cheese and margarine or butter; stir until melted. Pour onto a greased cookie sheet; spread into an 8-inch square about 1/2 to 3/4 inch thick. Cover and let stand at room temperature until cool and firm. (Or, chill overnight.)
To serve, cut firm grits into 16 triangles and arrange on the rack of an unheated broiler pan. Broil 4 to 5 inches from the heat for 4 to 5 minutes or until the surface is slightly crisp and beginning to brown. Makes 8 servings.