In a medium bowl, whisk together the tahini, lemon juice, shallot, & 1/4 cup warm water. Add more water as needed to get the sauce to have the thickness of creamy salad dressing. Season with a few twists of freshly ground salt & black pepper. Set aside.
Line a large cutting board with plastic wrap. Place one chicken breast on the board, cover with another sheet of plastic wrap, and use a meat mallet to pound the chicken into a cutlet that has a uniform thickness of roughly 1/2 inch. (If you don't have a meat mallet, you can use the bottom of a heavy frying pan to pound the chicken). Set the pounded chicken cutlet aside on a plate and repeat for the remaining 3 chicken breasts.
Crack the egg into a medium bowl, and whisk together with the Dijon mustard and sumac. Spread the panko on a large plate.
Season the chicken breasts with salt and pepper, then, one at a time, dip them in the egg mixture to coat. One at a time, press each chicken breast onto the plate of panko to coat well on both sides. Use your hands as needed to press the panko onto the chicken to create a uniform coating.
Next, drizzle 3 tablespoons of olive oil in a large nonstick skillet. You want the oil to evenly cover the pan, and provide a slight pool for the chicken to pan-fry in. Warm over medium-high heat.
When the oil is shimmering, place two of the breaded chicken cutlets in the warm skillet and cook for about 4 minutes per side, until crispy & golden. Remove from the pan & set aside. Add another splash of oil to the pan and repeat the process for the other two chicken cutlets.
Add the baby spinach to a large salad bowl. Drizzle with the remaining 2 1/2 tablespoons olive oil, and a splash of apple cider vinegar. Toss with your hands until evenly coated with dressing.
Place 1/4 of the spinach onto each serving plate. Top each with a chicken cutlet and drizzle with 1/4 of the tahini sauce. Season with additional black pepper & enjoy!