Clean the strawberries and cut the tops off. Leave to dry on paper towel in the refrigerator.
Spread the strawberry jam over two slices of the bread, leaving a 1 centimeter (1/2 inch) border. Place the remaining slices on top, to make two sandwiches.
Using a pastry cutter or the back of a knife, carefully press firmly just inside the crust, as you were cutting off the crusts, to seal the two bread slices together, encasing the strawberry jam.
Now cut off the crusts, just beyond where you sealed. (If the bread isn't sealed properly, you can pinch it closed with your fingers.)
Using a spatula, massage the mascarpone to soften it.
In a bowl or using an electric mixer, whisk the cream and sugar to soft peaks, then add the mascarpone and whisk to firm peaks.
Lay one of the strawberry sandwiches on a large square of plastic wrap, then spread with a layer of the cream.
Place three whole strawberries on the cream in a diagonal line, and a couple in the empty space.
Use the remaining cream to fill in the gaps between the strawberries and cover everything in cream.
Place the other strawberry sandwich on top and wrap tightly in plastic wrap. Use a market to draw where you've placed your line of strawberries.
Refrigerate for at least 3 hours, or overnight.
Keeping the plastic wrap on, cut along the line through the sandwich to divide it in two. Remove the wrap and serve.