Sprinkle salt evenly over both sides of eggplant slices; set aside for 5 minutes. Cut eggplant into cubes.

Cook and stir chicken, garlic, and soy sauce together in a large skillet over medium heat until the chicken is nearly cooked through, 7 to 10 minutes.

Stir mushrooms and black pepper into the chicken mixture; continue cooking until the mushrooms brown, 2 to 3 minutes.

Heat canola oil in a separate skillet over medium heat. Cook eggplant in hot canola oil until lightly browned; add to skillet with the chicken.

Stir spinach through the chicken and eggplant mixture; cook and stir until the spinach wilts, 2 to 3 minutes.