Toss the quartered strawberries and blueberries with 2 tablespoons of the sugar and lemon juice in a bowl. Refrigerate and let the berries macerate in the juice for 2 hours or longer.

Using an electric mixer, whip the mascarpone cheese until smooth. Add the heavy cream, the remaining 3 tablespoons of sugar and the vanilla extract and whip until the mixture thickens.

Strain the juice from the strawberries and blueberries into a bowl and add the cassis liqueur to the reserved juice. Add raspberries to the bowl of berries and toss together gently.

Dip each lady finger into the cassis liquid and turn over to lightly coat both sides. (Work quickly and don’t soak the lady fingers for too long – otherwise they will start to fall apart.) Layer half of the lady fingers into the bottom of an 8 x 8-inch casserole dish. (It’s ok if you need to break the last lady fingers to make them fit the pan.) Spread half of the mascarpone cream over the lady fingers and top with half of the macerated berries. Dip the remaining lady fingers in the cassis liquid and layer over the berries. Top with another layer of the mascarpone cream and the remaining berries.

Cover with plastic wrap and chill for at least 2 hours to overnight before serving. Cut into squares and serve chilled or if refrigerated longer than 2 hours, let it sit at room temperature for 20 minutes or so before serving.