Preheat the oven to 300 degrees F. In a large bowl, combine the chopped onion, beef, lamb, minced garlic, milk, breadcrumbs, oregano, thyme, fennel, coriander, egg and some of the salt and pepper and then mix together.
Transfer the meatloaf mixture to a food processor and process it to the consistency of a paste. Add a little cold water as needed to get the right consistency.
Fry up a small test batch and taste for salt, pepper and other seasonings. Adjust the seasonings as needed.
Transfer the pureed meatloaf mixture from the food processor to a greased loaf pan and pack it down tightly into the sides of the pan, making sure there are no cracks or air pockets. Bake until cooked through with an internal temperature of 155*F or higher, about 45 minutes to an hour depending on your oven.
When done, remove from the oven and drain off any fat. Place the loaf pan on a cooling rack, cover the top with a piece of aluminum foil and then weigh down with something heavy. You can use full soup or vegetable cans or wrap a brick with aluminum foil and use that. Allowing something heavy to sit on the loaf while it is still hot will make it compact and dense which will help is stay together for slicing. Allow it to rest like this for 20 to 30 minutes. After resting, remove the weights, pour off any accumulated grease, and run a knife around the edges of the loaf as needed. Then gently remove the loaf from the pan. The meat can then be sliced at that time if needed, however I prefer to wrap tightly with plastic wrap and then chill for several hours or overnight in the refrigerator to allow them to fully cool and firm up for easier slicing.