Using a mallet, tenderizer or thick-bottomed saucepan, flatten out the chicken thighs to approximately 1/4 inch thick.
Mix together the salt and sugar and sprinkle liberally all over the chicken thighs (you will only need about half of the mixture).
Place thighs on a small sheet pan and refrigerate for at least 2 hours and up to 24.
While thighs are curing, mix onions, water, butter, thyme and salt in a saucepan and bring the mixture up to a slight simmer over medium heat.
Continue simmering for 1 1/2-2 hours (until the water has almost all evaporated and the onions and butter have become silky). Remove the sprigs of thyme. Cover and place to the side.
After the soubise is on the stove, place the matzoh meal, nutmeg and pepper into a food processor.
Process the matzoh meal until it’s powdery. While the processor is still on, add the stock and the eggs. Process until everything is combined and the consistency of a smooth, thick paste (you may need to scrape down the sides of the food processor to get everything together).
Pour the spaetzle dough into a bowl and cover with plastic wrap. Let it sit for at least one hour.
Set up your frying station by turning your oven onto its warming setting.
Place three shallow dishes next to each other and going from left to right, fill them with: 1 1/2 cups of matzoh meal mixed with the thyme; the egg and Dijon mustard beaten together; and the last dish will hold the last 1/2 cup of matzoh meal (plain).
Place a pot of salted water over high heat. Bring to a boil.
Using either a colander or a spaetzle press, push the spaetzle dough into the salted boiling water. It will only need to cook for 2-3 minutes. Use a slotted spoon or a wire strainer to fish out your cooked spaetzle. Place cooked spaetzle into a bowl of ice water to cool and hold (it should take about three batches to cook).
After all the spaetzle is cooked and cooled, drain well, place in a bowl and coat with a couple tablespoons of olive oil.
To fry the chicken thighs:
Heat a non-stick pan up over medium heat and pour in 1/4 cup of the olive oil. Heat until the oil shimmers.
Run the chicken from right to left: Plain matzoh first, then the egg wash and then into the herbed matzoh and place into the hot oil. Do this again with a second thigh.
Fry on one side for about 3-4 minutes or until they are a dark golden brown. Flip and cook for another 3 minutes. Place the cooked chicken thighs onto a wire rack in a sheet pan and place into the over to keep warm. Repeat frying techniques with the remaining two chicken thighs.
To finish the dish:
Heat a non-stick pan over medium heat. Once hot, add a quarter of the spaetzle to the pan and sauté until it starts to brown.
Add 1/4 cup of the onion soubise and sauté until everything has heated through.
Serve spaetzle on a plate with some chopped fresh parsley over it and top with the chicken schnitzel.