In a deep saute pan over medium-high heat, add enough oil to completely coat the bottom of the pan. When the oil is hot, add the onion and saute until translucent, about 5 minutes. Add the ground turkey and season with cumin, salt, and pepper. Cook until the turkey browns and is cooked through. Remove the turkey mixture from the pan and wipe out the pan. Divide the turkey mixture among the tortillas and fold in half. Add more oil to the pan, and then brown both sides of the filled tortilla, in batches. Once the tacos are cooked place the cheese in the tacos, followed by the shredded romaine lettuce, and diced tomatoes. Top each tortilla with a dollop of sour cream and guacamole, if desired.