Use a 2-quart slow cooker. Put the onion, garlic, and cheeses into the slow cooker insert. Stir in the cornstarch and dry mustard. Crush the caraway seeds using a mortar and pestle and add to the pot. Pour in the beer.
Cover and cook on low for 2-4 hours, stirring every 30 minutes or so, as the ingredients melt into each other and thicken. Add salt and pepper, to taste, before serving. Leave the slow cooker plugged in and turned to low while serving.