1 Place cubed bread in a large bowl. Add milk and cover with plastic wrap; weight with a smaller bowl filled with water or a heavy plate. Chill in the refrigerator overnight.
2 Bring a large pot of salted water to a boil. Add spinach and cook until wilted; or about 1 minute. Drain and rinse with cold water to cool. Using your hands, squeeze the water from the spinach.
3 Using a food processor, puree the bread mixture until slightly chunky, or about 30 seconds. Add spinach, egg and 1/4 teaspoon salt; puree until the mixture forms a ball; about 2 minutes.
4 Transfer dough to a work surface and divide evenly into 4 pieces. Roll each piece into a 1/2-inch rope and cut into 1 1/2-inch pieces.
5 Bring a large pot of salted water to a boil. Add 10 pieces at a time and cook until tender; about 6 minutes. Using a slotted spoon; transfer dumplings to a large serving bowl.
6 Melt butter in a small saucepan over medium heat; add the sage leaves. Reduce heat to low and cook until butter is fragrant and the sage is infused, about 5 minutes.
7 Discard sage leaves, drizzle hot butter over the dumplings and gently toss to coat. Serve immediately with grated cheese.