In a small bowl, mix together the smoked paprika, garlic powder, onion powder, salt, and black pepper. Rub the olive oil over the flat iron steak, then coat both sides of the steak with the spice mixture. Let the steak marinate for at least 30 minutes at room temperature or up to 2 hours in the refrigerator for more intense flavor.

Preheat a grill or grill pan over medium-high heat. Once hot, place the marinated steak on the grill and cook for 4-5 minutes per side for medium-rare, or to your desired doneness. Remove from heat and let the steak rest for 5-10 minutes before slicing against the grain.

While the steak is resting, wash and peel the russet potatoes. Cut them into waffle shapes using a waffle fry cutter or a knife. Soak the cut potatoes in cold water for 30 minutes to remove excess starch, then drain and pat dry. Toss the potato pieces in cornstarch and Cajun seasoning until evenly coated.

Heat oil in a deep fryer or large pot to 350°F (175°C). Fry the potatoes in batches until golden brown and crispy, about 3-4 minutes per batch. Remove with a slotted spoon and drain on paper towels. Season with additional Cajun seasoning if desired.

In a small bowl, combine mayonnaise, minced garlic, lemon juice, Dijon mustard, salt, and pepper. Mix until smooth and well combined. Adjust seasoning to taste.

Slice the rested flat iron steak and serve alongside the Cajun-spiced waffle fries. Drizzle or serve the tangy garlic aioli sauce on the side for dipping.