Combine all pancake ingredients in an electric blender and blend at high speed for 30 seconds.
Scrape down the sides of the blender and blend again for 15 seconds, until the batter is smooth.
Allow batter to rest at room temperature for 30 minutes before using.
In a heavy skillet, melt 2 tablespoons butter over moderate heat; saute the onions until they are soft but not brown.
Add the mushrooms and cook for 10-15 minutes, stirring frequently, until most of the liquid in the pan has evaporated (Do not allow the mushrooms to brown).
Add the meat and cook, mashing it with a wooden spoon, until all traces of pink disappear.
Strain through a sieve and discard the liquid.
In a saucepan, melt the remaining 2 tablespoons butter over moderate heat.
Add the flour and mix well.
Pour in the stock and stir constantly with a whisk until the sauce comes to a boil and thickens.
Reduce the heat to low and let simmer for 5 minutes, stirring frequently.
Add the lemon juice, nutmeg, salt, and pepper.
Remove from the heat, and add to the meat mixture; mix well.
Taste for seasonings.
Cover loosely with foil to keep warm.
Heat a heavy 8" skillet over moderate heat until a drop of water steams for 2 seconds before it evaporates.
Brush the pan with half the melted butter and immediately pour in half the pancake batter.
Tip the pan gently from side to side to evenly coat the bottom of the pan.
Cook for 3 to 4 minutes, until the top of the pancake is dry and the bottom is golden brown.
Remove the pancake to a serving platter using a spatula, putting the "uncooked" side up. Repeat the process to make another pancake. Spread the meat filling evenly over the first pancake and place the second pancake "uncooked" side down on top of the meat mixture.