In a medium-sized saucepot over medium heat, add 3 tablespoons of unsalted butter and the remaining half of the onions. Season with salt and sauté for about 5 minutes until they soften.
Once lightly browned, reduce the heat to medium and continue cooking for another 10 minutes until the onions turn a deeper brown. This caramelization adds incredible flavor.
Add in 3 tablespoons of all-purpose flour to make a roux, stirring well to combine. Cooking the flour helps eliminate any raw taste.
Pour in beef stock and soy sauce, bringing it to a boil while stirring to thicken. The sauce should become glossy and rich.
Once thickened, reduce the heat to low and stir in the heavy whipping cream. Adjust the seasoning with more salt and pepper to taste.
Strain the sauce into a separate medium-sized pot and keep it warm over low heat.
In a non-stick pan, add 2 tablespoons of unsalted butter over low to medium heat. Once melted, add the half of the onions that remain and season lightly with salt. Sauté for about 10 minutes, stirring constantly until soft and golden.
Transfer the cooled onions to a large bowl. In the same bowl, whisk together 2 large eggs, ½ cup of whole milk, and 3 tablespoons of Dijon mustard. This mixture binds everything together.
Mix in 2/3 cup of breadcrumbs and 2 thick slices of crustless bread cut into cubes. Allow it to sit for 5 to 7 minutes until the bread absorbs the liquid and thickens the mixture.
Add the cooled sautéed onions, 2 pounds of ground beef, and season with salt and pepper. Mix thoroughly; you can fry a small test piece to check for seasoning.
Form the mixture into 18 golf-ball-sized meatballs. Ensure your hands are slightly damp to prevent sticking.
In a large skillet over medium heat, add enough cooking oil to coat the bottom and the remaining 3 tablespoons of butter. Once melted, add the meatballs.
Fry on all sides until they’re golden brown and cooked through, which should take about 12 to 15 minutes total. Keep an eye on them to ensure they don’t burn!
Serve the meatballs hot, drizzled with the warm brown gravy, and optionally garnish with finely minced fresh parsley. Enjoy the comforting flavors!