Preheat oven to 350 degrees F. In a small skillet over medium heat, melt the butter. Add onion and cook until softened, about 4-5 minutes. Stir in tomato paste and cook 1 minute. Add garlic and cook 30 seconds more. Remove from heat and let cool slightly.
In a large bowl, combine crushed crackers and milk. Let stand 2 minutes to absorb. Add ground chuck, ground pork, cooled onion mixture, egg, Worcestershire, Dijon, soy sauce, Parmesan, parsley, salt, and pepper. Mix gently with your hands until just combined. Do not overmix.
Line a sheet pan with parchment. Shape the mixture into a loaf approximately 8 inches long and 4-5 inches wide. Place on a wire rack set inside the sheet pan if available.
Whisk together all glaze ingredients in a small bowl. Spread half the glaze over the top and sides of the loaf.
Bake for 40 minutes. Remove from oven, spread remaining glaze over the loaf, and return to oven for 15-20 minutes more, until center reaches 160 degrees F.
Rest 10 minutes before slicing. Sprinkle lightly with flaky sea salt just before serving.