In a medium mixing bowl, combine the lean ground turkey, salt, black pepper, and egg. Add in the diced onion, minced garlic, jalapeño, lime juice, and feta crumbles. Mix thoroughly until well incorporated.

Divide the turkey mixture into six equal portions, each roughly 4.2 ounces. Roll each portion into a ball and then gently flatten them into patties, about ¾ inch thick.

Heat a large skillet over medium heat and drizzle in olive oil. Place three turkey patties in the skillet, ensuring enough space between them. Sear for about 5 minutes or until golden-brown.

After 5 minutes, flip the turkey patties gently and continue cooking for another 5 minutes. Ensure the internal temperature reaches 165°F.

While the turkey patties are cooking, prepare the cilantro crema by blending fresh cilantro, lime juice, sour cream, and a pinch of salt in a food processor until smooth.

Toast the hamburger buns in the same skillet for about 1-2 minutes, cut side down, until golden and slightly crispy.

To assemble, spread a layer of cilantro crema on each toasted bun, add a patty, more crema, avocado slices, and crispy onion rings. Top with the other half of the bun.