Bring 1 pound of grilling steaks to room temperature for at least 1 hour. Salt and pepper both sides lightly and allow to stand an additional 20 minutes.
Preheat a gas grill for medium heat. Alternatively, if using a charcoal grill, prepare coals on half the grill for medium heat and set grate over both hot and cool sides of grill.
Halve the baby Dutch potatoes lengthwise and place in a microwave-safe bowl. Cover with paper toweling and heat on high for 8 to 10 minutes. Potatoes should be steaming and very tender. Drain if any moisture has accumulated in the bowl.
Coat steak, potatoes, and onions with oil and 1 tablespoon of a good Creole or Cajun seasoning. Line grill grates with aluminum foil. Place steak, potatoes, and onions on grill. Grill 3 to 4 minutes on each side or until potatoes are marked, onions are charred, and steak is 125°F for medium rare; warm red center. **SEE NOTES FOR ADDITIONAL TEMPERATURES AND TIMES
Transfer steak to a rimmed cutting board. Tent tightly with foil to maintain heat and let stand 5 minutes before slicing. Temperature will rise 5 to 10°F during the rest time so be sure to factor this into your doneness calculations.
In a food processor, combine the mayonnaise, garlic, paprika, vinegar, and remaining Creole or Cajun seasoning. Rough chop the green onions into roughly 1/2-inch segments. Stir into the bravas sauce.
Toss potatoes in mayonnaise mixture or serve on the side, about 1 tablespoon per person. Serve sliced steak with patatas bravas.