Although this dessert is best made immediately before serving, it can be assembled before dinner and refrigerated for up to one hour. If possible, use seedless jam, which won't require the straining in the first step.
Pass jam through a fine sieve; discard solids.
With a mixer fitted with the whisk attachment, beat cream and sugar until soft peaks form.
Gently fold jam into cream to form swirls. Divide mixture among 4 cups, garnish with berries, and serve immediately.