Preheat a grill.
Saute the shallots and garlic in olive oil until the shallots are translucent. Add the spinach and continue to saute until the spinach is tender and reduced in size. Add the cinnamon and pine nuts, mix, and set aside.
Depending on the size of the oysters, arrange 1 to 2 oysters on reserved half-shells. Place a portion of the spinach mixture over the oysters. Spoon the bechamel sauce topping over the spinach mixture. Sprinkle crumbled feta cheese over the bechamel.
Grill on the half-shell over a medium high heat fire for approximately 5 to 10 minutes, or until oysters are done. Serve immediately.
Melt butter in saucepan and whisk in flour until well blended. Cook for 1 to 2 minutes to remove the starchy taste of the flour. Gradually add the milk, stirring, until the mixture comes to a boil. Season with salt, pepper, and nutmeg. Simmer until the sauce thickens. Beat the egg yolk slightly. Add the hot, thickened sauce to the egg yolk, stirring well. Pour the egg yolk mixture back into saucepan, stirring constantly. Add the crumbled feta cheese and stir constantly until cheese is melted and fully incorporated. Remove sauce from heat and use immediately.