Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius).
Melt the butter in a pan over medium heat.
Add the chopped onion and cook until translucent, approximately 5 minutes.
Combine the ground turkey, sautéed onion, eggs, cheddar cheese, salt, pepper, tomato sauce, Worcestershire sauce, and crushed saltine crackers in a large bowl.
Mix everything together until well combined. Avoid overmixing the meat.
Transfer the meat mixture to a loaf pan, shaping it into a loaf shape.
Drizzle the top of the meatloaf with ketchup.
Bake the loaf pan in a preheated oven for approximately one hour, or until the internal temperature reaches 165 degrees Fahrenheit (74 degrees Celsius).
Remove the baked meatloaf from the oven and allow it to settle for a few minutes.
Slice the meatloaf and serve it warm.
Enjoy your delicious Joanna Gianes meatloaf!