To halve the chicken breasts crosswise as opposed to pounding them out, use one hand to press lightly on top of the breast and the other to hold the knife and gently saw back and forth till you've made your way down the breast and are left with two pieces.
In place of the mustard-mayo mix, you could do a more traditional flour-egg-crumb station. To do this, whisk two eggs with 1/4 cup flour and 2 tablespoons of water.
Preheat the oven to 375ºF. Place pumpkin seeds on rimmed sheet pan. Transfer to oven and cook for 5 minutes—seeds should barely take on any color. Set pan aside to cool.
Meanwhile, prepare the chicken breasts: If the tenderloin is still intact, slice it off and set it aside. At this point, you can either pound the chicken breasts to a 1/2-inch thickness, or you could cut each breast in half crosswise, to create 4 thin pieces—this is my preference. (See notes above for guidance.)
In a medium bowl, stir together the mayonnaise and mustard. Place the chicken breasts and tenders in the bowl and rub the mixture all over them to coat them evenly.
Place the cooled pepitas, smoked paprika, cumin, and cayenne in a resealable bag. Seal the bag, pressing to release air before you close it completely. Holding a wine bottle by the neck, gently tap the bottom of the bottle against the seeds until they are crushed, but not uniformly fine like bread crumbs. I like a mix of full seeds, coarsely crushed seeds and finely crushed seeds. Transfer the seeds to a small rimmed sheet pan or other similar vessel.
Transfer the breasts and tenders one by one to the vessel with the seeds, turning each to coat until each is evenly coated in seeds. Season both sides of each breast with salt and pepper to taste.
Heat a large sauté pan over medium-high heat. Add 3 to 4 tablespoons of oil—you want a thin layer. When it shimmers, lower each breast into the pan and turn the heat down to medium. Cook for 2 to 3 minutes, flip, and cook for 2 to 3 minutes more or until evenly golden.
Transfer breasts to a platter to rest. Let rest 5 minutes before slicing and serving.