To prepare the beef: In a medium bowl, stir together 1 1/2 teaspoons sugar, cornstarch, 1/2 teaspoon pepper, garlic, 1 1/2 tablespoons oyster sauce, soy sauce and fish sauce. Taste and, if a saltier finish is needed, add up to 1 1/2 teaspoons more oyster sauce. Add the beef, toss to coat well, and let marinate for 20 minutes at room temperature. Keep the canola oil nearby.
To make the salad: Rinse the onion in a strainer under cold running water for about 10 seconds, then set aside.
In large bowl (suitable for tossing the salad), whisk together 1 1/2 teaspoons sugar, salt, 4 grinds pepper, vinegar and 2 tablespoons water. Add the onion, top with the greens and add the mint and tomatoes, if using; but don't toss the salad.
Set a large skillet that can get very hot over high heat, and add enough of the canola oil to film the bottom. When the oil is shimmering, carefully add the beef, spreading it out over 1 layer, and cook for 3 to 4 minutes, shaking the pan every 30 to 60 seconds to sear beef on all sides; it should be medium-rare. (If you want to minimize mess, cover the pan with a splatter guard, and flip the meat with a spatula.) Remove from heat.
Quickly toss salad and transfer everything to a platter or serving dish. Pile the cooked beef and its juices on top, and serve immediately. At the table, ceremoniously combine all the ingredients.