Optional: Remove the tendons from the tenderloins. To do this, place the end of the tendon between the tines of a fork that is resting against the tenderloin. Grab the end of the tendon with a paper towel (because it is slippery) and pull it through the fork. This will remove the tendon without removing much of the chicken. (see video above recipe)
Whisk together water, 3 Tbsp fine sea salt, peppercorns, garlic, tamari, and bay leaf in a large mixing bowl. Add the chicken tenders, cover, and refrigerate for at least two hours or up to 12 hours.
Combine parmesan, garlic powder, pepper, 1/2 tsp fine sea salt, onion powder, cumin, parsley flakes, and paprika in a small mixing bowl and set aside.
Remove the chicken tenders from the brine and pat dry with paper towels. Strain the brine through a strainer, discard the solids, and wipe the large mixing bowl dry with a paper towel. Return the chicken to the bowl.
Add the 3 tbsp avocado oil to the chicken and stir to coat the chicken evenly.
Add the spice mixture to the chicken and stir to coat evenly.
Spray a large cast iron skillet lightly with avocado oil spray and heat over medium to medium-high heat until you see the first tiny wisp of smoke.
Add half of the chicken tenders to the pan and cook for 4-5 minutes per side, only flipping once with a small metal spatula. As the chicken cooks, you will see the edges of the chicken turning white. It is ready to flip when all of the tenders have white extending up over the edges and slightly onto the top of the tenders all the way around. Adjust the heat as necessary to make sure the tenders get brown, but don't burn. When they are finished, transfer them to a large plate and cover them with another large plate. Let rest for at least five minutes before checking the fattest tender to make sure it is cooked through.
Repeat the previous step with the remaining tenders, adding them to the same plate as the first batch and letting them rest for five minutes before cutting into them.