Have grill pre-greased and preheated to medium-high.
Place steak in 1 bowl and onions in another. Using a knife, make a paste by combining garlic and 1/4 teaspoon salt. Whisk together garlic paste, rice vinegar, soy sauce, 2 tablespoons of canola oil, cumin, and ground black pepper. Divide marinade between steak and onions. Refrigerate for 1/2 hour.
Before grilling, allow steak and onions to come to room temperature. Place steak on grill and cook about 4 minutes per side for medium-rare. Discard marinade. When done, let steak rest before slicing.
Place onions face down on grill and cook until slightly soft, about 6 minutes per side. When done, cut onions into wedges.
Sprinkle paprika over potatoes and toss with remaining canola oil. Place on grill and cook until soft inside, about 6 minutes per side. Season with salt when done.
In a large bowl combine steak and juices, onions, potatoes, tomatoes, cilantro, and reserved marinade. Toss gently and reseason with salt and pepper if needed. Serve warm.