Place the chicken thighs, 2 cups of the chicken broth, and the salt in a medium pot. Bring to a gentle simmer over medium heat. Cover loosely and cook until the chicken reaches 165°F (74°C) in the thickest part. Transfer the chicken to a plate and reserve the broth in the pot. When cool enough to handle, cut or shred the chicken into bite-size pieces.
Heat the red palm oil in a Dutch oven or large soup pot over medium heat. Add the onion and cook, stirring often, until softened. Stir in the tomato paste and cook until it darkens slightly and smells less raw.
Pour in the reserved chicken broth and the remaining 1 cup broth. Stir in the ground crayfish, if using, bouillon, and cayenne. Bring the mixture to a gentle simmer.
In a small bowl, mix the ground egusi with the water to make a thick, scoopable paste. Spoon the paste into the simmering broth in small mounds. Do not stir for the first 8 minutes so the egusi can set into soft dumplings.
Gently stir and break up some of the egusi mounds, leaving a few larger pieces if you like more texture. Simmer gently, stirring occasionally, until the soup looks thick and the ground melon seeds no longer taste raw.
Return the chicken to the pot. Stir in the spinach a handful at a time just until wilted. Taste and add more salt only if needed. Serve warm with pounded yam, fufu, eba, or rice.