Preheat oven to 150C. Tear the sourdough into small pieces, place on a baking tray and bake until dry (14-18 minutes). Cool briefly, then process to coarse crumbs in a food processor.
Increase oven to 180C. Melt butter in a saucepan over low heat and sauté onion, garlic and thyme until onion is softened (14-16 minutes). Add prunes and stir occasionally until softened slightly (5 minutes), then add breadcrumbs, liver and sage, season and stir occasionally until well combined and liver is just cooked (2-3 minutes).
Place stuffing in a small (about 16cm x 20cm) baking tray lined with baking paper and bake (or refrigerate at this point if making ahead) until golden and crisp on top (35-45 minutes). Serve stuffing with roast goose.