Preheat oven to 375°F (190°C).
In a large bowl, mix together ground beef, onions, milk, eggs, salt, pepper, thyme, garlic powder, and crushed crackers until well combined.
Shape the mixture into a loaf and place it in a greased baking pan.
In a small bowl, whisk together ketchup, brown sugar, and mustard. Spread this mixture over the meatloaf.
Bake for 1 hour, or until the meatloaf is cooked through and the glaze is caramelized.
Let it rest for 10 minutes before slicing.