Combine the whipping cream, mascarpone cream and sugar in a bowl. Whip with an electric mixer until thick and creamy. Add the vanilla and whip until combined.
Cover with plastic wrap and chill in the refrigerator.
In a medium saucepan, combine 4 cups of Saskatoon berries, 1¾ cups of water and the sugar bring to a boil over medium high heat. Boil for 15 minutes until the berries break down and the juice is a dark rich color. Strain the mixture through a fine sieve to remove any pulp then transfer the mixture back to the same saucepan. Stir the cornstarch and lemon juice into the remaining ¼ cup of water and add to the berry mixture in the saucepan. Cook the berry mixture until thickened.
Meanwhile, measure the flour into a large bowl, then slowly whisk in the half and half cream. Whisk in the salt, eggs, cinnamon, nutmeg, vanilla and sugar until smooth. Place the bread slices in the cream egg mixture until well-saturated, turning if necessary. Heat a lightly oiled skillet over medium heat. Remove the soaked bread slices letting the egg cream mixture drip off, then place in the heated skillet and brown on each side until golden brown and crisp. Assemble one slice of French toast on a plate, spoon some mascarpone cream mixture on top of the toast slice, top with another slice of French toast. Top with the Saskatoon berry sauce and garnish with any remaining Saskatoon berries.