►Grate Gruyère, cheddar, and fontina into a large mixing bowl. Add cornstarch and toss to coat.

►Transfer cheese mixture to a medium saucepot, add evaporated milk, and stir to combine. Cook over low heat, whisking constantly, until melted and beginning to bubble, about 5 minutes. Cook and whisk for 2 additional minutes, or until mixture thickens and comes together.

►Remove pot from heat and thin sauce with milk if desired (start with 2 tablespoons and increase if necessary). Use immediately, or transfer to to a pint-size container and cool completely. Cover and store in refrigerator for up to 1 week or in freezer for up to 3 months. When ready to use, thaw sauce in refrigerator overnight if frozen. Pour sauce into a saucepot and add 2 tablespoons whole milk. Warm gently over low heat, stirring occasionally. Add more milk to thin if desired.