Heat the oil in a large skillet over low heat. Add the bell pepper, onion, celery, and garlic. Cover and cook, stirring occasionally, until the vegetables are soft and translucent, about 7 minutes.
Push the vegetables to one side of pan. Raise the heat to medium. Add the meat to the side without the vegetables and saute until almost cooked through, about 7 minutes.
Stir the meat and vegetables together. Add the vinegar and stir. Add the tomato paste and sugar, if using, and cook, stirring occasionally, until lightly browned, about 4 minutes.
Add the tomatoes, nutmeg, salt, and black pepper. Reduce the heat to medium-low, and simmer until thick, about 3 minutes.
Toast the buns. Divide meat between buns, garnish with lettuce and red onion, if desired.
Recipe by Go Dairy Free at https://www.godairyfree.org/recipes/healthy-turkey-sloppy-joes