Preheat your oven to 180°C (350°F). Grease a 23x33 cm (9x13 inch) baking dish with a little butter. As you work, the oven will begin to warm the kitchen, setting the stage for the cosy work ahead.

Brown the turkey: Heat the olive oil in a large frying pan over a medium-high heat. Add the ground turkey and cook, breaking it up with a wooden spoon, until it is no longer pink and begins to brown. You'll hear a satisfying sizzle, and the meat will turn a deep golden brown. Season with a pinch of salt and pepper, then transfer the turkey to a plate using a slotted spoon, leaving any juices in the pan.

Soften the aromatics: Reduce the heat to medium. Add the chopped onion to the same pan and cook for 5-6 minutes, stirring occasionally, until it becomes translucent and smells sweetly fragrant. Add the minced garlic and cook for another minute until you can smell its pungent aroma. The onions should be soft and golden, not browned.

Make the béchamel sauce: Melt the butter in the pan over a medium heat. Once it has stopped foaming, whisk in the flour to form a paste, or roux. Cook this for 1-2 minutes, stirring constantly, until it smells slightly nutty and has turned a pale biscuit colour. Slowly pour in the milk and chicken stock, a little at a time, whisking continuously to prevent lumps. The sauce will thicken and become glossy as it comes to a gentle simmer. You'll know it's ready when it coats the back of a spoon—a visual cue that tells you it's perfectly thick.

Season the sauce: Remove the pan from the heat. Stir in the dried thyme, grated nutmeg, and half of the grated cheddar cheese. Whisk until the cheese has fully melted into the sauce, creating a smooth, creamy mixture. Taste and adjust the seasoning with salt and pepper. The sauce should be rich and velvety.

Assemble the layers: Spread a thin layer of the cheese sauce (about 3-4 tablespoons) over the bottom of the prepared baking dish. Arrange a third of the potato slices in overlapping layers over the sauce. Scatter half of the cooked ground turkey evenly over the potatoes. Pour a third of the remaining sauce over the turkey. Repeat with another layer of potatoes, the rest of the turkey, and another third of the sauce. Finish with a final layer of potatoes and pour the remaining sauce over the top, making sure all the potatoes are covered.

Bake to golden perfection: Sprinkle the remaining cheddar cheese and the grated Parmesan over the top. Cover the dish tightly with foil and bake for 40 minutes. After 40 minutes, remove the foil and bake for another 20-25 minutes, until the top is deeply golden and bubbling, and the potatoes are tender when pierced with a knife. You'll hear a gentle, contented bubbling sound as it bakes, and the aroma of cheese and thyme will fill every corner of your home.

Rest and serve: Remove the dish from the oven and let it stand for 10-15 minutes before serving. This crucial rest allows the sauce to settle and thicken, making it easier to slice and serve. Garnish with fresh chopped parsley just before bringing it to the table.