In a large bowl, combine the ground chicken or turkey, feta, oats, diced red onion, dill, cumin, ½ teaspoon turmeric, 1 teaspoon salt and ½ teaspoon pepper. Gently mix with your hands until just combined. Form the mixture into 16 to 18 meatballs, each about the size of a golf ball.
In a large Dutch oven or pot, heat the olive oil over medium-high heat. Add the grated onion, 1 tablespoon turmeric and the red-pepper flakes. Cook, stirring occasionally, until the onion is golden brown and cooked through, 4 to 6 minutes. Add the chicken broth or water and bring to a simmer. Add the meatballs and simmer until they’re cooked through and firm, 7 to 9 minutes.
Stir in the spinach and cook until wilted, 1 to 2 minutes. Remove from heat and stir in the lemon juice. Taste and season with salt and pepper if needed. Serve hot with lemon wedges and additional red-pepper flakes.