Well, it doesn't sound too hard to make...

I guess take 1 portabella mushroom, remove the stem, place a slice of tomato or 2 slices of sun dried tomato, then some thin slices of brie. Put it under the broiler until the cheese melts, and then top with the sauce:

1 Cup butter at room temperature
3 Tbsp water
3 egg yolks
salt
white pepper

Cut butter into 1" pieces. In a small, heavy saucepan add 2 tablespoons water and reduce over high heat to about 1 tablespoon. Remove from heat and add 1 tablespoon cold water. Beat egg yolks lightly and whisk into water. Over low heat, whisk the butter - one piece at a time - making sure that each piece of butter is melted before adding more. Continue to whisk the sauce until it is thick. Add salt and pepper to taste.

Makes 8 servings. .1 carb per serving.

The mushroom plus the sauce would yield around 4.7 g that's using 1 thick slice of tomato and 1oz of brie per mushroom. Portabella's are high in carbs compared to the other mushrooms out there.