Heat 1 tablespoon canola oil in a medium to large over medium-high heat. Add the bell peppers and onions. Season with a little salt and pepper and saute until soft, about 5 minutes.
While the vegetables are cooking, divide the hoisin sauce, soy sauce, mirin, and sesame oil in half in 2 separate bowls. In one bowl, add the cornstarch. This bowl will be used for cooking, the other bowl will be used for a topping. Stir both bowls well and set aside.
Add the garlic and ginger and saute for another 3 minutes. Remove the vegetables from the skillet.
In the same skillet, heat the remaining canola oil and add the beef. If it is shaved thinly, it will cook quickly, in about only 1-2 minutes.
Return the vegetables to the pan and add in the bowl of hoisin sauce, soy sauce, mirin, sesame oil, and cornstarch. Mix thoroughly and remove from the heat.
To serve, place 2-3 pieces of Boston lettuce on a plate. Place a small amount of the beef and vegetable mixture on the lettuce. Top with sesame seeds and green onions, and spoon some of the sauce you aside on top of the beef.
How to Cut the Beef: using a freshly sharpened knife, slice very thin pieces off the sirloin, hold the knife on a 45° angle. Essentially, you are slicing the meat, but extremely thin.
How to Toast Sesame Seeds: Toasting the sesame seeds brings out the flavor which makes them taste better. Place the sesame seeds in a nonstick skillet over medium-low heat. Stir frequently to avoid burning. The seeds are done when they begin to look oily. The time may vary, so keep a close eye on them. But usually it takes about 7-10 minutes.