The soaking water for the nuts is used again to proof the yeast, which infuses the entire loaf with nutty flavor.
Preheat oven to 350°.
Spread hazelnuts in an even layer on a baking sheet. Cook at 350° for 10 minutes or until light brown. Remove from heat; transfer to a small bowl. Pour boiling water over hazelnuts; cover and let stand 20 minutes. Strain hazelnut mixture through a sieve into a bowl, reserving liquid and nuts. Spread nuts in an even layer on a double thickness of paper towels; pat lightly with additional paper towels to dry. Set aside. Place reserved liquid in a small saucepan; heat until warm (100° to 104°). Transfer reserved liquid to a large bowl. Add sugar and yeast, stirring well with a whisk; let stand for 5 minutes.
Lightly spoon flour into dry measuring cups; level with a knife. Add 2 3/4 cups flour and salt; stir until a soft dough forms. Turn dough out onto a floured surface. Knead until smooth and elastic (about 8 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel tacky). Cover dough; let rest 10 minutes. Uncover; knead in reserved hazelnuts.
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in size. (Press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down; cover and let rest 5 minutes. Shape dough into a 9-inch oval; place on a baking sheet sprinkled with cornmeal. Lightly spray surface of dough with cooking spray; cover. Let rise in a warm place (85°), free from drafts, 30 minutes or until doubled in size.
Preheat oven to 375°.
Bake at 375° for 30 minutes or until browned on bottom and sounds hollow when tapped. Cool on wire rack.