Heat oven to 375°F. Line rimmed baking sheet with foil. In bowl, combine ketchup, brown sugar and 1 teaspoon soy sauce. In large bowl, whisk together eggs, balsamic vinegar, remaining tablespoon soy sauce and ½ teaspoon each salt and pepper; stir in panko and let sit 2 minutes. Add onion and parsley and mix to combine. Add beef and mix just until incorporated. Transfer mixture to prepared baking sheet and shape into 9- by 3½-inch loaf. Brush loaf with ketchup mixture. Bake until internal temp registers 150°F, 40 to 45 minutes. Let rest 5 minutes before slicing. Serve with salad if desired.