Melt the butter in a saucepan over medium heat.
Sprinkle the flour over the butter and whisk until no lumps of flour remain.
Turn the heat down to medium-low and stir continuously with a whisk for 2 minutes. If you see the mixture is starting to turn a little darker, turn the heat to low. You want a white sauce, not brown.
Add the milk (or half and half), heavy cream, and nutmeg and stir well.
Raise the heat to medium-high and continue to whisk as the sauce comes to a gentle boil and thickens.
Once the sauce is a smooth, thickened consistency, turn off the heat and add the cheddar and fontina by handfuls, whisking to combine after each addition. Add salt and pepper, to taste.
Turn the heat to low and stir gently to combine and heat through. Turn off the heat until ready to use.
Now here’s the fun… Add approximately 2 heaping tablespoons of the ricotta mixture into each shell, taking care to not tear the shells. Place the filled shells as you fill them in the greased 9x13” baking pan.
Evenly pour the warm sauce over the shells.
Bake, uncovered, for 25 to 35 minutes or until the edges are bubbly and the sauce is lightly browned.
Remove the dish from the oven and allow it to sit for 10 to 15 minutes. Sprinkle with the parsley, if desired.
Serve warm.