Grate half a yellow onion until you have about ¼ cup, then combine it in a mixing bowl with kosher salt, almond flour, and red curry paste. Mix these ingredients thoroughly, then fold in the ground chicken gently. Form the mixture into meatballs about 1 to 1.5 inches in diameter.

In a large pot, combine chicken stock with an equal part of water and half of the coconut milk. Heat the mixture over medium-high heat until it begins to simmer gently. Add minced garlic, grated ginger, and optional lemongrass. Let the broth simmer for about 10-15 minutes.

Stir in the remaining red curry paste, coconut aminos, and a splash of fish sauce to the simmering broth. Gently add the formed meatballs in batches and cook for roughly 10-12 minutes, turning occasionally until fully cooked.

Remove lemongrass and any large aromatics from the pot. Stir in the remaining coconut milk, lime juice, and spinach. Cook for an additional 2-3 minutes until spinach is just wilted.

Ladle the soup into bowls, garnishing with fresh herbs like cilantro or mint. Optionally add pre-cooked rice noodles before serving.