Combine the chicken and eggs in a large bowl. Season all over with salt.
Place the garlic, Thai chili peppers, shallot, and 2 scallions in a food processor and pulse until minced.
Add the minced aromatics to the chicken along with lemongrass paste. Use your hands to combine. Add ½ cup panko or breadcrumbs and mix to combine. If the mixture is too wet, add an additional ¼ cup of breadcrumbs. Mix until combined and let stand 10 minutes.
Heat 1 tablespoon oil over medium heat in a wide pot. Add a teaspoon of the meatball mixture and fry until cooked through. Taste and adjust seasonings if needed.
Fry the meatballs:
Form the mixture into meatballs and fry for 3–4 minutes per side until browned all over. They need not be cooked through. Transfer to a plate.
Cook the mushrooms and onions:
If the pot seems dry, add another tiny drizzle of oil. Add the onions and mushrooms and cook for 7–8 minutes. Stir the mushrooms regularly and continue cooking until they brown around the edges. Season with salt.
Sizzle the paste:
Add the green curry paste and let it sizzle in the pot for 30 seconds. Toss to coat the mushrooms and onions in the paste. Pour in the water or broth.
Simmer the meatballs:
Add the chicken meatballs to the pot and bring to a boil. Reduce heat and simmer 25 minutes or until meatballs reach 165ºF. Taste and season the broth to your preferences.
Finish the broth:
Mince the remaining 1 scallion, keeping the white and green parts separate.
Stir the coconut milk and white parts of the scallion into the broth. Simmer 5 minutes.
To serve:
Ladle the broth over cooked rice noodles. Garnish with the minced scallion greens, lime wedges, and crispy shallots. Add a drizzle of chili oil if you like.