Preheat the oven to 350 degrees F. Line a large rimmed baking sheet with heavy-duty aluminum foil for easy cleanup, folding the excess foil over the edges. Set aside.
Combine all the “Meatballs" ingredients in a large bowl, gently mixing with your hands or a spatula. (I’ve seen “chefs” use a stand mixer, but I would not do that. Too much handling can overwork the proteins in the meat and result in tough meatballs. Mix gently to keep tender meatballs.)
Form the meat into uniform balls of equal size. This will allow the meatballs to cook evenly. I use my #20 disher (yellow handle), 1.625 ounces.
Bake in the preheated oven for 20 to 30 minutes, or until a probe thermometer reads 165 degrees F when inserted into the center of a meatball. (My meatballs take about 27 minutes.) Remove them from the oven.
While the meatballs are baking, make the sauce. In a 3-cup (or larger) measuring cup, add the beef stock and cornstarch. Stir to dissolve. Set aside.
In a medium skillet, heat the oil over medium heat. Once hot, add the onion and sauté until just softening and turning translucent, approximately 5 minutes.
Add the garlic, lemongrass, ginger, and chiles and stir constantly for 30 seconds.
Stir the stock-cornstarch mixture and add to the pan, along with the coconut milk. Heat through to dissolve the milk solids.
Add the white pepper, salt, curry leaves, brown sugar, and fish sauce. Stir and bring to a gentle simmer over low heat, stirring to dissolve the sugar.
Cover and simmer for 20 to 30 minutes, stirring often, to allow the flavors to meld together and slightly thicken.
Remove the curry leaves and discard. Remove the chiles and discard. Taste and adjust as desired.
Add the meatballs, tossing them gently to coat with the sauce. Heat them through if necessary.
Add the green onions and cilantro. Serve very warm over rice.