In a pan over medium heat, warm the coconut oil and add the sliced onions. Season with a bit of salt. Cook slowly, stirring occasionally, until they turn golden brown and soft. This should take about 15-20 minutes. Add the balsamic vinegar and celery seed and stir well, letting the flavors meld for another 2-3 minutes. Remove from heat and let cool slightly. Puree with an immersion blender.
Prepare the Meat Mixture:
In a large bowl, combine the ground meat, almond flour, tomato paste, egg, salt, pepper, garlic powder, and onion powder. Add in the caramelized onion puree and mix everything together until well incorporated.
Shape and Bake:
Instead of forming the mixture into a loaf, spread it evenly into a 9x13 glass baking dish. This allows the meatloaf to cook faster and ensures even cooking. Bake at 375°F for 40 minutes, or until the internal temperature reaches 160°F.
Rest and Serve:
Let the meatloaf rest for about 10 minutes before slicing. Using a baking dish makes it easy to cut into squares, making serving much simpler. Transfer to a serving plate.