Heat a griddle over high heat, or preheat a gas grill to high. Lightly brush the beef with oil and season with salt and pepper. Prepare an ice bath.
Sear the beef on the griddle or grill on all sides until browned, about 2 minutes per side. Plunge into the ice bath and allow to cool, 3 to 4 minutes.
Remove the filet and drain well on paper towels, patting the meat dry.
In a 2-quart saucepan, heat 3/4 inch of oil over medium–high heat until shimmering hot. Add the garlic and fry until golden and crispy, about 1 minute. Remove with a slotted spoon. Drain on paper towel.
In a small bowl, whisk together the onion, oil, lemon juice, vinegar, soy sauce, ginger, and garlic until combined.
In another small bowl, whisk the soy sauce, vinegar, and bonito flakes, if using.
With a sharp carving knife, thinly slice the beef and arrange on 10 dinner plates. Spoon some of the onion ponzu on top of the beef, then drizzle on some tataki dressing, and top with the garlic chips, scallions, and chives.