Spray a non stick skillet with cooking spray. Saute onions and mushrooms over medium high heat until liquid has evaporated. Reduce to low; stir in crab meat and sour cream until heated through (sour cream should not boil); set aside. Beat 2 eggs with 2 tbsp (30 mL) water; season with salt and pepper. Spray an 8 inch ( 20 cm) non stick skillet with cooking spray. Heat skillet over medium
high heat. Pour in the egg mixture. As mixture sets at the edges, with spatula, gently push cooked portions towards centre. When eggs are almost set on surface but still look moist, sprinkle 2 tbsp (30 mL) cheese over centre of omelette. Lower heat to medium low and place 1/3 cup (75 mL) crab meat mixture over half of the omelette. Fold the omelette in half and slide onto a warm plate. Repeat procedures to make 3 more omelettes. Garnish with shrimp halves.