Thinly slice the red onion and submerge the slices in the pickled jalapeño brine.
Cut the avocados into thick wedges or sturdy slices so they remain intact during the breading process.
Beat the eggs thoroughly in a shallow dish.
In a separate shallow dish, mix together the cornmeal, panko breadcrumbs, cayenne pepper, salt, and garlic powder.
Lightly sprinkle salt over the avocado slices.
Coat each avocado piece in the whisked egg, letting any excess drip off.
Dredge the avocado in the cornmeal-panko coating, pressing lightly to ensure it sticks evenly.
Heat oil in a skillet set over medium heat until the bottom is thinly coated.
Add the breaded avocado slices to the hot pan in a single layer, taking care not to crowd them.
Pan-fry the avocado, flipping gently, until the exterior becomes crisp and golden brown.
Heat the tortillas in a warm skillet until soft and flexible.
Layer each tortilla with torn baby spinach and Monterey Jack cheese.
Place the crispy pan-fried avocado slices over the cheese and greens.
Garnish with the jalapeño-marinated red onions.
Serve immediately while the avocado remains crunchy and the cheese is melted.
