Make your brine by adding everything but the beef cheeks to a stock pot. Bring to a simmer, stirring along the ways, ensuring you dissolve the salt and sugar. Once dissolved, remove from the heat, and let it cool.
Add the beef cheeks to a large sealable bag and pour in the cooled brine. Seal and place in the refrigerator overnight.
The following day, pour out the brine, and add the beef cheeks to a large saute pan or dutch oven. Add about 2 cups of water or beef stock, bring to medium heat, cover, and cook for about 3 hours. Reduce the heat to a medium-low once it comes to a boil.
After 3 hours, remove the beef cheeks onto a plate, and let them cool. Once cooled, slice and then chop the cheeks.
When you are about ready to serve, heat a large skillet on medium-high heat and add in the oil. Add as much beef cheeks as you want, and cook until those beef cheeks get a nice sear on them. You want to get that great crispiness on it.
When you are ready to serve, add the beef cheeks to a warm and toasted tortilla, top with fresh cilantro, minced shallot, a bit of avocado and a squeeze of lime juice. Repeat.