Preheat the oven to 180°C (356°F) with top and bottom heat.
Heat a frying pan over medium heat with a little olive oil. Sauté the chopped leeks and onion for 5–6 minutes until softened and lightly caramelized. Season with salt and pepper and set aside to cool.
Lay the deboned chicken flat on a work surface and season lightly with salt and pepper. If needed, gently pound thicker areas with a meat mallet so the chicken is an even thickness for rolling.
Spread the cooled leek and onion mixture evenly over the chicken, leaving a small border around the edges. Arrange the tomato slices and cheese sticks in an even layer on top of the leeks.
Carefully roll the chicken tightly from one end and tie it in about five places with kitchen twine to secure the filling.
In a small bowl, whisk together the smoked paprika, turmeric, mustard, 1 tbsp olive oil, salt and pepper to make the coating. Brush the mixture all over the chicken roll.
Heat a large skillet over high heat with a little oil and sear the chicken roll for about 1 minute per side until browned on all sides.
Place the seared roll on a baking tray lined with parchment paper, cover with parchment and aluminum foil, and bake for 1½ hours. Remove the covers and bake an additional 20–30 minutes until cooked through and nicely browned.
Let the roll rest for 10 minutes, remove the twine, slice into portions and serve.