Divide the mostly thawed frozen berries, separating 1/4 into a small bowl. Set aside.
Puree the remaining 2 cups of frozen raspberries in a blender with 1 tablespoon of granulated sugar. Taste test and add 1 more tablespoon if desired. Blend until smooth.
In a large mixing bowl, combine the cream and vanilla extract. Add the 1/4 cup confectioners sugar, one tablespoon at a time, tasting after each addition.
Whip cream with an electric stand or hand mixer on high until stiff peaks form and cream is spoonable and holds its shape.
Spoon half the raspberry purree onto the whip cream. With a large soup spoon or spatula, gently fold the pureed raspberries into the whipped cream mixture, leaving visible swirls. Gently fold in the rest of the puree. Be careful not to overmix.
Spoon the whipped cream into 4 cups or bowls. Combine the fresh and frozen raspberries. Spoon it evenly on top and sprinkle with sliced almonds. Serve immediately.