Preheat the oven to 170 degrees Celsius. Spread the diced bread in a baking tin in a single layer, drizzle with a little olive oil and bake for about 15 minutes or until golden brown and crispy. Remove from the oven and leave to cool slightly. If preferred, ready-made croutons can be used.
Meanwhile, boil the pasta in abundant salted water according to the instructions on the packet. At the same time, in another pan, sauté the garlic in the olive oil over a low heat.
When the pasta is ready, drain reserving one cup of the cooking water and pour this into the pan with the garlic and oil. Stir in the yogurt and, if the mixture seems a little dry, add some of the reserved water. Season with salt and pepper and garnish with the croutons, a little oregano and, if using, grated cheese.
Variation: instead of mixing the yoghurt with the pasta, top each plate of pasta with a dollop of yogurt.