Peel and chop the onion. Sauté the onion in a saucepan with a few tablespoons of olive oil until slightly wilted (about 3–4 minutes).
Add tomato puree and season with salt. Cook over medium heat for about 10 minutes. Finally, season with a few basil leaves.
While the tomato sauce cooks, bring some sheep meat broth to a boil in a large pan. Traditionally, boiling the broth adds more flavor to the carasau bread. You can also boil it in salted water or other broths.
Once boiling, lower the heat slightly and immerse carasau bread sheets for a few seconds until soft. Use a slotted spoon to drain the boiled carasau bread and place on a plate or bowl.
Spread a layer of tomato sauce over a serving plate. Cover with a layer of carasau bread. Top with more sauce and sprinkle with Sardinian pecorino cheese. Repeat layering until you have 3–4 layers of bread.
In the simmering broth, crack an egg into a small bowl. Whisk the broth to form a vortex and gently slide in the egg. Cook for about 2 minutes until set, stirring gently if the vortex slows. Use a slotted spoon to place the poached egg on top of the layered bread. Sprinkle with more pecorino cheese.
Your pane frattau is now ready to serve, a delicious blend of textures and flavors that embodies the essence of Sardinian cuisine.