In a large bowl, combine the ground chuck, ground pork, ground chicken, and onion. Mix by hand and set aside.
Tear the trimmed white bread into small pieces. Place into a bowl and soak the bread in the milk.
Add the beaten eggs, ketchup, mustard, and Worcestshire sauce to the bowl with the soaked bread. Mix lightly by hand, and then combine with the meat. Add salt and pepper. Continue to mix by hand until all ingredients are blended together.
Shape the meat mixture into a loaf and place into a baking pan that has sides. Place the meatloaf into the oven and cook for one hour at 400 degrees.
While the meatloaf is cooking, prepare the mushroom gravy on the stove. In a medium pan or pot, add the oil and butter and allow it to heat for 2 minutes on medium heat.
Add the onions and garlic. Sweat over low heat until tender and translucent. Add the mushrooms and turn up to medium heat. After about 5 minutes, add the flour. Saute until browned and tender.
Add the beef broth and heavy cream, and simmer for about 10 – 15 minutes. Season with salt and pepper to taste.
Turn off the heat and let the gravy sit until the meatloaf has finished cooking.
After one hour, take the meatloaf out of the oven and let it rest for 5 - 10 minutes. Slice the meatloaf and serve with mushroom gravy on top.