Heat olive oil in a skillet on medium heat and sauté the onion and the garlic until translucent. Add the butterflied chicken breast.
Sprinkle with the spice mixture on both sides of the chicken. Brown the chicken slightly on both sides.
Pour in the water, cover and allow it to simmer on medium heat for about 20 minutes. Add more water if needed to keep it simmering while covered. Flip over half way through cooking.
Cook until the chicken is completely done and has and internal temperature of 165°F.
Remove from the skillet and place on a cutting board. Allow to rest and cool for 10 minutes. Shred using two forks.
Preheat oven to 350°F.
Melt butter in a sauce pan over medium heat. Quickly add the flour and stir quickly using a whisk. Begin adding the milk slowly while stirring and breaking down the lumps using a whisk. Sprinkle in the salt and stir. Slowly blend in the Gruyere cheese.
Toss in the jalapeño and the nutmeg. Whisk and reduce until the mixture is slightly thick or nappe consistency. Taste for salt level and add as needed.
Pour ½ of the sauce evenly on the bottom of a baking dish.
Fill the baking dish with filled rolled tortillas.