Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9x5-inch loaf pan.

Heat 1 tablespoon oil in a skillet over medium heat. Add mushrooms, bell pepper, and onion; cook and stir until just soft, 3 to 4 minutes. Add garlic and cook until fragrant, about 30 seconds. Set mixture aside and let cool.

Meanwhile, mix bread crumbs, parsley, egg, and Italian seasoning together in a large bowl. Add cooled veggie mixture; mix well. Gently mix in ground pork and ground beef. Place mixture into the prepared loaf pan and form into a loaf shape.

Mix tomato sauce, Dijon mustard, brown sugar, and vinegar together in a bowl. Set glaze aside.

Bake meatloaf in the preheated oven for 45 minutes. Remove from the oven and brush glaze over loaf. Return to the oven and continue to bake, brushing with glaze every 10 to 15 minutes, until no longer pink in the center, about 30 minutes more. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C). Remove from the oven and drain excess oil.