Place a greased 10-in. springform pan on a double thickness of heavy-duty foil (about 18 in. square). Securely wrap foil around pan.
Place hazelnuts in a food processor; cover and pulse until coarsely chopped. Set aside 1/4 cup for garnish. Add graham crackers and sugar to food processor; cover and process until finely chopped. Add butter; process until blended. Press onto the bottom and 1 in. up the sides of prepared pan. Place pan on a baking sheet. Bake at 325° for 10 minutes. Cool on a wire rack.
In a large bowl, beat the ricotta, cream cheese, sour cream and sugar until well blended. Beat in flour and 2 tablespoons liqueur. Add eggs; beat on low speed just until combined. Pour into crust. Place springform pan in a large baking pan; add 1 in. of hot water to larger pan.
Bake at 325° for 1-1/2 hours or until center is just set and top appears dull. Remove springform pan from water bath. Cool on a wire rack for 10 minutes. Carefully run a knife around edge of pan to loosen; cool 1 hour longer. Refrigerate overnight.
Toss peaches with remaining liqueur; arrange over top of cheesecake. Sprinkle reserved hazelnuts in the center. Remove sides of pan. Yield: 16 servings.