Cook brown or white rice as directed. Note: brown rice can take up to 50 minutes to cook so plan accordingly.
Cook ground beef with chopped onions and 2-3 Tbsp taco seasoning in a large skillet over medium. Drain, if needed. (Or thaw and heat frozen taco meat!)
Drain corn. Drain and rinse black beans, if using canned. Heat up both in a small sauce pan.
Prepare guacamole by mashing avocado with the ground cumin and lime juice, season to taste with salt and pepper.
Layer plates with rice, taco meat, beans, and corn. Add guacamole, cheese, and salsa to each plate.
Serve Mexican Stack-Es with a cilantro garnish.