Heat a large skillet over medium-high heat. Add the ground turkey and diced onion and cook, stirring frequently, until the turkey is browned and the onion is soft, about 5 minutes.
Add the taco seasoning and follow the packet directions (usually adding a bit of water). Reduce heat to low and simmer the mixture for about 10 minutes until flavors meld.
Spread about 1 tablespoon of refried beans on one half of a corn tortilla. Spoon a heaping portion of the turkey mixture on the other half and sprinkle with shredded cheese; do not fold yet.
Heat a clean large skillet over medium and add enough vegetable oil to lightly coat the bottom. When the oil is hot, place the prepared tortilla flat in the pan and cook about 30 seconds to make it pliable.
Using tongs, carefully fold the tortilla in half. Continue to cook 2–4 minutes per side until the taco is golden brown and crispy.
Remove tacos to paper towels to drain and repeat with remaining tortillas, adding more oil as needed.
While still warm, open each taco and add any optional fillings (lettuce, tomato, lime juice, cilantro, sour cream, avocado, hot sauce) before serving.
