Lay a warm tortilla flat on a clean surface and spread a thin layer of Dijon mustard or cream cheese if using.
Place 2-3 slices of turkey in the center of the tortilla, overlapping slightly to cover a small rectangular area.
Top the turkey with a generous sprinkle of cheese, about 2 slices or 1/4 cup shredded, covering the filling evenly.
Gently roll the tortilla from one edge, pressing firmly to create a tight cylinder, sealing the edge with a damp finger if needed.
Repeat with remaining tortillas and fillings, assembling all roll-ups ready for cooking.
Heat the oil or butter in a large non-stick skillet over medium heat until shimmering and fragrant, about 1-2 minutes.
Place the roll-ups seam-side down in the hot skillet, gently pressing down with a spatula, and cook for 2-3 minutes until golden brown and crispy on the bottom.
Carefully turn the roll-ups to another side and cook for another 2-3 minutes, until all sides are golden and cheese is melted inside.
Remove the roll-ups from the skillet and let rest for 1 minute to set the filling.
Slice each roll-up diagonally with a sharp knife if serving as appetizers, or serve whole for a quick, handheld meal.
Serve immediately, perhaps with a squeeze of lemon or a side salad for a complete meal.
