Cut the apple into thin slices onto a plate, and drizzle with about 2 teaspoons of the lemon juice.
Cover the plate tightly, and refrigerate.
Pour the remaining lemon juice onto a second plate. Peel and slice the avocado, then place the slices in the lemon juice, then turn them over until they are well coated. Set aside.
Heat the olive oil in a medium-sized skillet or a ridged grill pan; add the onion and cook over high heat for 5 minutes. Sprinkle in the cumin seeds and walnuts, turn the heat down to medium, and cook with minimal stirring for another 2 to 3 minutes, or until the seeds and nuts give off a toasty aroma. Be careful not to let them burn!
Add the onion mixture to the spinach, and toss until thoroughly mixed. The spinach will wilt upon contact with the hot oil and onion. To speed this process along - and to be sure you include every last drop of the flavorful oil - you can add some of the spinach directly to the pan and swish it around a little, then return it to the bowl. Sprinkle in the salt as you toss.
Gently mix in the avocado, including all the lemon juice, and the apple. Grind in a generous amount of black pepper, and serve right away, garnished with lemon wedges to squeeze on top, if desired.