Arrange a rack in the middle of the oven and heat to 350°F. Line a 9x5-inch loaf pan with aluminum foil.

Heat the oil in a large, heavy-bottomed pot or Dutch oven over medium-high heat until shimmering. Add the onions and baking soda and cook, stirring every few minutes with a wooden spoon, for 15 minutes. Continue cooking, stirring more frequently and scraping up the browned buildup that forms at the bottom of the pan, until the onions are a deep golden-brown, 10 to 15 minutes more.

Place the breadcrumbs in a large bowl, pour in the milk, and stir to combine. Add the caramelized onions (reserving some for garnish, if you want), garlic, Worcestershire, salt, and pepper and stir to combine.

Add the ground meat and eggs, and use your fingers to combine the meat and flavorings. Handle the meat gently and do not overmix. Transfer the mixture to the prepared pan and gently press down into an even layer.

Bake until the middle of the loaf registers at least 165°F on an instant-read thermometer, 55 to 60 minutes.

Remove the meatloaf from the oven and set the oven to broil. Sprinkle the reserved caramelized onions and Gruyère evenly over the top of the meatloaf. Broil until the cheese is bubbly and browned around the edges, about 2 minutes.

Let the meatloaf cool in the pan for 15 minutes. Remove from the pan by grasping and lifting it out with the foil to a clean cutting board. Drain off the excess grease. Slice and serve.