Freeze the tenderloin until firm, 20 to 30 minutes. Cut the tenderloin crosswise into 1/3-inch-thick medallions. Slice each medaillon into 1/3-inch-wide strips.
Combine the pork, 1 tsp fish sauce, and the soy sauce in a small bowl.
Whisk the remaining 2 tbsp fish sauce, the chicken broth, lime juice, brown sugar, and cornstarch in a measuring cup.
Combine garlic, lemon grass, red pepper flakes and 1 tbsp of the oil.
Heat 1 1/2 tsps oil in a nonstick 12-inch skillet or in a wok over high heat until smoking; add half of the pork to the skillet and cook, stirring occasionally and breaking up the clumps, until well browned, about 2 minutes. Transfer pork to a bowl. Repeat with an additional 1 1/2 tsps oil and the remaining pork.
Add 1 tbsp oil to the now-empty skillet or wok; add the asparagus and cook, stirring every 30 seconds, until lightly browned, about 2 minutes. Add the water, cover the pan, and lower the heat to medium. Steam the asparagus until tender-crisp, about 2 minutes; transfer the asparagus to the bowl with the pork.
Add the remaining 1 1/2 tsps oil to the skillet or wok; add the onion and cook, stirring occasionally, until beginning to brown and soften, about two minutes.
Clear the center of the skillet; add the garlic and lemon grass mixture to the clearing and cook, mashing the mixture with a spoon, until fragrant, about 1 minute, then stir the mixture into the onion. Return the pork and asparagus to the skillet and toss to combine. Whisk the sauce to recombine, then add to the skillet/wok; cook, stirring constantly, until the sauce is thickened and evenly distributed, about 30 seconds. Transfer to a serving platter and sprinkle with the thai basil. Serve immediately.