Set a heavy bottomed, medium-sized saucepan over medium heat. Add butter and flour and cook, stirring until roux is well blended and bubbling.

Remove from heat and add stock all at once, whisking quickly to incorporate well and prevent lumps. Return to heat, stirring frequently until thickened.

In a small bowl, whisk egg yolk with 1 tablespoon of the cream and set aside. Add the remaining cream to the sauce in the pot and stir until heated through. Reduce heat to low.

Slowly add a ladle of hot cream sauce to the egg yolk, whisking constantly. This will temper the egg yolk so it mixes in without getting lumpy. Once it's been incorporated, pour the yolk mixture into the hot sauce and mix well.

Add lemon juice, whisk and then add both kinds of cheese. Stir once, then let melt, stirring occasionally. Season with salt and pepper if desired. Mornay sauce is finished.

Pre heat broiler or oven to 500'F. Prepare oysters. If large, cut into bite-size pieces and return to half-shell or place in ramekins. One large or 2 small oysters per ramekin or one oyster per shell. Place all shells or ramekins on a baking sheet.

Top oysters with Mornay sauce to cover. Sprinkle lightly with paprika if desired and place sheet of oysters in the oven under the broiler. Broil 4-6 minutes, until turning golden brown and bubbly. Turn broiler off and let oysters cook 1-2 more minutes before removing from oven.