Preheat the oven to 220°C (428°F) with top and bottom heat, or 200°C (392°F) with convection.
Wash and dry the waxy potatoes, then slice them into thin fan-like cuts using a wooden spoon placed alongside as a guide to control depth. Melt the butter, then line a baking tray with baking paper, arrange the potato slices, drizzle with melted butter, and season with salt and pepper.
Bake the potatoes for 30-35 min until crispy and golden.
While the potatoes bake, stir the yogurt until smooth, mix in the Italian herbs, and season with salt and pepper.
Trim and clean the spring onions and red peppers, cut the peppers into strips, cut the spring onions into rings, peel and finely chop the garlic, wash and pat dry the chicken breast fillets, cut them into strips, and season with gyros spice, salt, and pepper.
Heat the olive oil in a large pan over medium-high heat. Fry the chicken strips for 3-4 min until golden and cooked through (internal temperature 74°C / 165°F).
Add the spring onions, peppers, and garlic to the pan and fry for 1-2 min more, then adjust the seasoning to taste.
Serve the chicken strips with the crispy potatoes and yogurt dip on the side.