This is a rich, hearty dish with plenty of typical Thai flavors.
Any kind of beef may be used, but flank steak works particularly well because it is easy to cut it across the grain, which helps keep the meat from falling apart during stir-frying and produces a tender result.
Be sure to serve plenty of rice, because it helps moderate the hot chilies without detracting from the flavor.
You may reduce the number of chilies by up to one half, but traditionally this dish should have a rich, hot chili flavor.
Slice the beef across the grain into strips 1/4 inch thick and 2 to 3 inches long.
Pound or grind the chilies, garlic, and onion to a coarse paste in a mortar or blender.
If you use a blender you may need to add the oil to aid in grinding.
Heat a wok, add the oil, and swirl it over the surface of the pan.
(Do not add more oil if you have ground the chilies, onion, and garlic in oil.
) Add the paste from Step 2 and stir-fry until it is light golden.
Add the beef and stir-fry until it is a uniform tan color, but do not overcook it.
Add the fish sauce, sugar, water, and mint (or basil) leaves.
More water may be added if the sauce is too dry.
There should be about 1/2 to 3/4 cup sauce, depending on how much water you added.
Ahead of time note: The dish may be prepared a day in advance to this point.
To do so, proceed through Step 5, omitting the mint or basil leaves.
When you are ready to serve, heat the mixture and add the leaves.
If the meat has absorbed the liquid, add just enough warm water to bring it back to the original consistency.
Arrange a single layer of lettuce leaves in a serving bowl and put the beef mixture over them.
Serve the beef immediately or keep it warm while preparing other dishes.
Serve with rice.
(Serves 4 to 6)