Sprinkle gelatin over water and let stand until softened. Warm 1 cup sour cream over low heat, add gelatin and stir constantly until gelatin melts, 1 to 2 minutes. Remove from heat and add mayonnaise, lemon juice, onion, sugar and hot pepper sauce, blending well. Place caviar in strainer and rinse carefully with cold water. Shake strainer to remove excess water.
Reserve 1 tablespoon caviar. Add remaining caviar to sour cream mixture, stirring gently. Season to taste with salt and white pepper. Turn into greased 2-cup mold or into 8 to 10 small individual molds and chill until firm, about 3 to 4 hours.
Unmold on serving plate. Place about 6 tablespoons sour cream on top of large mold or 1 tablespoon on each individual mold. Place some of reserved caviar on top of sour cream. Surround with parsley and accompany with toast rounds.