Bring the stock to a boil in a medium saucepan. Once boiling, add the grits and whisk constantly until they are fully mixed into the stock, about a minute or two. Reduce the heat to low and cover, stirring once or twice more, until the grits are thicker and creamy, about 5 minutes. Stir in the butter, grated fontina, blue cheese salt and pepper. Taste and season more if desired – this will depend on the salt in your stock and cheese.