Prepare an ice bath with a large bowl filled with ice and a little water. Set aside.
Set up a double boiler with a heat-proof bowl set on top of a 4-quart pot with 1 to 2 inches of simmering water. The water should not touch the bottom of the bowl. Off of the heat, add the sugar and egg yolks and whisk together until they lighten. Whisk in the prosecco and rum, if using.
Put the bowl over the simmering pot and whisk constantly until the mixture thickens and becomes thick and frothy, 3 to 5 minutes. You'll know it's done if it forms a ribbon (figure 8) when drizzled back into the bowl. Remove from the heat and set in the ice bath to cool.
Meanwhile whip the cream to soft peaks and set aside. When the zabaglione is cooled, add a spoonful of the cream to the zabaglione to lighten it and stir with a whisk. Gently fold in the remaining cream in 2 intervals.
Put the halved strawberries into 4 dessert bowls. Spoon over the zabaglione and serve immediately.