Preheat the grill or grill pan over medium-high heat.
In a small bowl, combine the ground cumin, chili powder, paprika, garlic powder, salt, and pepper. Rub the spice mixture onto both sides of the flank steak. Let marinate for at least 20 minutes or up to a day.Alternatively, mix all the spices with the ground beef, cook in a medium heat skillet while breaking up into smaller pieces until no longer pink. In a large skillet, heat the vegetable oil over medium-high heat. Add the diced onion and cook until softened and lightly browned, about 3 minutes. Remove from heat and set aside.
Grill the marinated flank steak for 2-3 minutes per side or until desired doneness. Remove from the grill and let it rest for a few minutes before slicing it into thin strips.
While the steak rests, prepare the guacamole. Cut the avocados in half and remove the pits. Scoop the flesh into a bowl and mash with a fork until desired consistency. Add the diced tomato, red onion, lime juice, chopped cilantro, salt, and pepper. Mix well.
Combine the sparkling water with the lemons and limes. Stir and let sit in the refrigerator, if desired.
Warm the corn tortillas on the grill or stovetop over medium heat for about 30 seconds on each side.
To assemble the street tacos, place a few slices of grilled flank steak on each warm tortilla. Top with the sautéed onions and chopped cilantro. Serve with lime wedges on the side.
Serve the Mexican street tacos with a side of tortilla chips and guacamole, and glasses of lemon-lime sparkling water, served over ice if desired.