Place a deep non-stick pan over medium to high heat and add the butter to melt.
Once the butter has melted, add the flour and stir well until fully combined with the butter.
Add the milk and yogurt, whisking continuously until the mixture becomes smooth and uniform.
Keep stirring until it comes to a boil, then remove from the heat. Add the cooked pasta, 2/3 of the graviera, and the feta. Season with salt and freshly ground pepper, and mix well.
Transfer the mixture to an ovenproof dish and sprinkle the remaining cheeses on top.
Place the nuggets on a baking tray lined with parchment paper and bake them together with the pasta in a preheated oven at 180°C for 22–24 minutes.
