Make the Meatballs:
HEAT olive oil and add onions in a large deep sauté pan. Sauté on medium heat until the onions are translucent, about 5 minutes.
ADD garlic and sauté for an additional 30 seconds.
DECREASE heat to low.
ADD diced celery and cook until soft, about 5 minutes. Let cool for 5 minutes.
PREHEAT oven to 400ºF and line a baking sheet with parchment paper.
COMBINE ground crackers, parsley, ground beef, egg, all spice, salt, pepper and cayenne, if using, in a large mixing bowl.
MIX until combined.
ADD onion and celery mixture and mix once more. Set aside.
FORM 1-inch meatballs using your hands and arrange on the prepared baking sheet.
BAKE for 25 minutes.
Make the Gravy:
ADD beef stock, Dijon mustard and cream cheese in a blender. Blend until smooth.
TRANSFER to a medium saucepan. Bring to a boil. Once boiling, reduce heat to medium low and cook for 15 minutes to reduce.
ADD tapioca with ¼ cup of the sauce in a small bowl.
WHISK until all lumps are eliminated.
POUR into saucepan. Stir well.
SIMMER for another 5-8 minutes or until the sauce coats the back of a spoon, stirring occasionally.
SERVE with gravy and garnish with fresh parsley once meatballs are finished cooking.