Heat the butter in a saucepan, add the onion and fry gently for 5 minutes until softened but not browned. Add the potato, toss in the butter then cover and fry gently for 10 minutes, stirring occasionally until just turning golden around the edges.
Add the stock, season with salt and pepper and bring to the boil. Cover and simmer for 10 minutes or until the potatoes are tender. Cool slightly then purée in batches in a blender or food processor with the herbs.
Pour back into the saucepan, reheat then taste and adjust the seasoning if needed. Serve in mugs with toasted bacon sandwiches.