Make this ahead, store it in the freezer and pull out to serve at the last minute. If desired, garnish with fresh raspberries and a dollop of whipped cream.
Lightly butter the base of a 9-inch (2.5 L) springform pan; set aside. In medium bowl, combine Oreo crumbs and butter until moistened. Press evenly and firmly in bottom of pan. Place in freezer.
In food processor, purée raspberries until smooth. Place purée in a fine-mesh sieve and press firmly to push raspberries through. Reserve purée; discard seeds.
In heatproof bowl, whisk together egg yolks, ½ cup (125 mL) sugar, lemon rind and lemon juice. Place over simmering water and cook, whisking constantly, until translucent and thick enough to coat the back of a spoon, about 5 to 8 minutes. Remove from heat. Whisk raspberry purée into egg mixture, cover loosely and refrigerate until cold, about 30 minutes.
In a clean medium bowl, using an electric mixer, beat egg whites, salt and cream of tartar until foamy; beat in remaining ¼ cup (50 mL) sugar, 1 tbsp (15 mL) at a time, until stiff peaks form. Gently fold egg whites into raspberry mixture.
In a separate bowl, using electric mixer, whip cream until firm peaks form. Fold into raspberry mixture. Spread filling over crust. Cover with foil and freeze for at least 8 hours or overnight.
Thirty minutes before serving, remove springform ring and place mousse on large plate. Place in refrigerator to soften before cutting into wedges.
Tip: Wrap mousse in springform pan well in plastic wrap, then in foil and freeze for up to 2 weeks.
Tip: A serrated knife works well to cut. Run under hot water and wipe with a warm, damp cloth, as needed between cuts, to help keep slices neat and tidy.
Tip: Make individual mousses by spooning filling mixture into small dishes, such as ramekins or into paper lines muffin pan.
Tip: Omit lemon rind and replace lemon juice with milk or water for a non-citrus variation.