Heat a large, 10 to 12-inch skillet or fry pan over medium. Once warm ,add the jasmine rice and toast, stirring often until lightly brown all over and the rice is nutty and aromatic, about 7 minutes. Turn off heat, transfer rice to a blender (or spice grinder in two batches) and grind until you have a grainy powder. Transfer to a bowl and set aside. Wipe the large griddle pan clean.
If needed, cut the flank steak into sizes that will fit in the pan above then season the steak with salt and pepper and set aside. Heat the canola oil in the wiped clean large griddle pan over a high. Once the oil is hot and shimmery, sear the steak in batches, undisturbed until they have developed a well-browned crust, about 4 minutes. Turn the steak over, and cook for an additional 3 minutes or to 145 degrees for medium-rare. Transfer to a plate to rest.
With a sharp chef’s knife, slice off the bottom fibrous part of the lemongrass stalks (about 1/2 inch) and then slice each stalk into extremely thin rings (seriously, as thin as possible) and transfer to a large mixing bowl with the onion, radishes, Thai chile, five spice, lime juice, fish sauce and set aside.
Slice the steak against the grain into long thin steps and add to the lemongrass bowl. Toss very well so the steak is coated evenly with the lemongrass, spices and chile evenly. Add the cilantro, mint and 3 tablespoons of the rice powder and toss again.
Transfer the steak salad to a serving platter with white rice and the remaining toasted rice powder on the side.