Season the chicken breast on both sides with sea salt, black pepper, and garlic powder.
Heat avocado oil in a skillet over medium heat and sear the chicken for 5 to 7 minutes per side until fully cooked through.
Remove the chicken from the heat, let it rest for 2 minutes, and then slice it into thin strips.
Whisk the egg whites briefly and pour them into a small, lightly greased non-stick pan over medium-low heat.
Cook the egg whites until firm and set, then fold them into a square shape to fit the dimensions of the muffin.
Toast the English muffin until the edges are golden and crisp.
Spread the mashed avocado on the bottom muffin half, then layer with fresh spinach, sliced chicken, and the egg white patty before serving.