In a medium bowl, combine sliced strawberries and granulated sugar. Let sit for 30 minutes to macerate.
Arrange half of the cake cubes in the bottom of a 3-quart glass trifle bowl.
Drizzle 1/4 cup of cream sherry evenly over the cake cubes.
Distribute half of the macerated strawberries and their juices over the cake layer.
Pour 1.5 cups of vanilla custard over the strawberries, smoothing the surface with a spatula.
Repeat layers with the remaining cake, sherry, strawberries, and custard.
In a chilled bowl, whip heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
Top the final custard layer with the whipped cream, creating decorative peaks.
Refrigerate for at least 4 hours to allow flavors to meld and layers to set.
Garnish with toasted almond slivers immediately before serving.