It has to be something that can be made in less than 6 hours...
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Apple turnovers using puff pasty and ice cream.
amysarah is a trusted home cook.
Maybe shortbread, since it takes place in Scotland? You could do some berries and cream as well - not really in season now, but that would fit with the book's summer house setting, and is very traditional British.
Chris is a trusted source on General Cooking
I do hope you're doing the boeuf en daube? Madeleines (keeps that French theme of the boeuf en daube going and also resonates with the remembrances of the last section of the book, also reminding us of Proust's influence on Virginia Woolf). And mandarin oranges. They echo the orange flavoring from the daube, while being a nice, light counterbalance to a heavy stew. I love that book. Have a wonderful time.
If i remember correctly, the heroine drowns herself (as did VW) so maybe serve a dessert that is part of a meal that would be the last dessert you'd want to make sure and have if you knew it was to be your last. That would be different for everyone, but since it is your designation, make your very favorite dessert.
No drowning in this one, though Mrs. Ramsey, who was so proud of here boeuf en daube, does die. But I do think you're on the right track, what you want is a dessert that you can serve and be able to look at your guests, thinking how much they are enjoying both the food and the company. Something that will get you feeling that it's your favorite contribution and also will make your guests think that dinner is "a triumph!"