To prepare gnocchi, combine 1/4 cup water and potatoes in a microwave-safe dish. Cover and microwave at HIGH 10 minutes or until tender. Drain potatoes; mash potatoes with a potato masher. Cool 5 minutes.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine potatoes, 3/4 cup flour, hazelnut meal, and next 3 ingredients (through egg yolk); mix well. Knead until smooth (about 2 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands.
Divide dough into 4 equal portions. Shape each portion into a 12-inch-long rope. Cut each rope into 20 pieces; roll each piece into a ball. Working with one dough piece at a time (cover remaining dough to prevent drying), using your thumb, roll dough piece down the tines of a lightly floured fork (gnocchi will have ridges on one side and an indention on the other). Place gnocchi on a lightly floured baking sheet.
Bring 1 gallon water to a boil in a large stockpot.
To prepare glaze, melt butter in a small saucepan over medium heat. Add 1 minced garlic clove to pan; cook 30 seconds. Add broth; bring to a boil. Stir in sage, 1/4 teaspoon salt, and pepper. Remove from heat.
Add half of gnocchi to boiling water; cook 3 minutes or until done (gnocchi will rise to the surface). Remove cooked gnocchi with a slotted spoon; place in a colander. Repeat procedure with remaining uncooked gnocchi. Serve drizzled with warm glaze.