In a medium bowl, combine the grated onions, panko breadcrumbs, 3/4 teaspoon kosher salt, and all the remaining ingredients, leaving out the ground beef and oil.

After 5 minutes, add the ground beef gently and mix just until combined, being careful not to overwork the mixture.

Divide the mixture into about 18-22 meatballs, each roughly 1 1/2 tablespoons in size.

Add the tablespoon of olive oil to a large skillet over medium-high heat.

Once hot, place the meatballs into the skillet in a single layer and cook for 6-9 minutes, turning halfway through, until browned.

Transfer the meatballs to a plate, adding more oil if necessary.

In the same skillet, add 1 tablespoon of butter and let it melt.

Add chopped mushrooms and cook for 3-4 minutes until golden brown.

Add sliced onions and sauté for 3 minutes until softened.

Incorporate minced garlic and cook for 30 seconds until fragrant.

Add remaining butter, allowing it to melt, then stir in flour, cooking for 1 minute.

Gradually pour in beef stock while stirring to avoid lumps, creating a smooth gravy base.

Stir in Worcestershire sauce, Dijon mustard, and soy sauce, allowing gravy to come to a simmer for about 3 minutes to thicken.

Return meatballs to the skillet with any juices and cook for 4 minutes.

Taste the gravy and adjust seasonings as needed before serving.