Melt the butter with olive oil in a 26 cm / 10-inch Dutch oven over medium heat. Add the sliced leek and cook 5 to 6 minutes until softened and translucent.
Add the garlic and sage leaves, cook 1 minute until fragrant.
Add the spring greens in batches, stirring until wilted, about 3 to 4 minutes.
Add the torn bread to the pot, stirring to coat it in the fat and vegetables. If your bread is fresh rather than stale, dry it in a 150 C / 300 F oven for 10 minutes first, or it will turn gummy instead of creamy.
Pour in the broth and bring to a gentle simmer. Cook uncovered 15 to 18 minutes, stirring often, until the bread breaks down into a thick porridge that mounds softly on a spoon.
Season with salt, pepper, and a pinch of nutmeg.
Make 4 shallow wells in the pancotto and crack an egg into each. Cover the pot and cook over low heat 3 to 4 minutes, until the whites are fully set and the yolks are still soft.
Remove from the heat and scatter the cubed Casera over the surface. Let it melt for 1 to 2 minutes without stirring, so the eggs stay intact.
Ladle into bowls with one egg each. Sprinkle with Parmesan and black pepper, drizzle with a little olive oil, and serve right away.