Heat oil in a large skillet over medium. Add onions and 1/2 teaspoon salt, and cook, stirring often, until deeply golden brown, very soft and jammy, 35 to 45 minutes. Add 2 tablespoons sherry to skillet and cook, stirring constantly, until mostly evaporated, 15 to 30 seconds. Remove from heat and set aside until cool enough to handle, about 15 minutes.

Preheat oven to 350°F with rack in the center position. Finely chop 1/2 cup caramelized onions and place in a large bowl; set the remaining caramelized onions aside.

Add panko, eggs, onion soup mix, chopped thyme, pepper, 1 tablespoon Worcestershire sauce, and remaining 1 teaspoon salt to bowl with caramelized onions; stir to combine. Add ground beef and ground pork and gently mix with clean hands until evenly combined.

Divide meat mixture into 6 equal portions (about 8 ounces each); form into 5- x 2-inch meatloaves. Place meatloaves on an aluminum foil-lined baking sheet.

Bake the meatloaves for 20 minutes. Meanwhile, stir together ketchup, remaining 1 tablespoon Worcestershire sauce, and remaining 2 teaspoons sherry in a small bowl until smooth. Remove baking sheet from oven, brush ketchup mixture evenly over tops and sides of meatloaves. Return to oven and continue baking until an instant-read thermometer inserted into thickest portion of meatloaves registers at least 160°F, 5 to 10 minutes.

Remove baking sheet from oven and turn oven to broil with rack about 7 inches from heat source. Top meatloaves evenly with reserved caramelized onions, and sprinkle evenly with Gruyère (about 1/4 cup each). Broil until cheese is melted and starting to brown in spots, 3 to 5 minutes.

Garnish with additional fresh thyme and black pepper, and serve with mashed potatoes.