This is a twist on the classic French onion soup. It never
ceases to amaze me how wonderfully complex the flavour of onion can
be when it's given the time to fully unfurl. Don't try to shorten
the cooking time - patience brings rewards.
Melt butter in a large wide saucepan over low heat, add onions and speck, cover and cook, stirring occasionally, until onions are very soft (20-25 minutes). Remove lid and cook uncovered, stirring occasionally, until starting to caramelise (50 minutes to 1 hour). Add ½ cup stock and simmer until almost evaporated (4-5 minutes), then repeat three times. Add remaining stock, sage and bay leaf, season to taste, bring to the boil, then reduce heat to low and simmer uncovered, scraping the base of the pan occasionally to remove any caramelised bits, until thick (8-10 minutes).
Preheat oven to 200C. Ladle soup into 1½-cup ovenproof bowls and transfer to an oven tray. Place a slice of toast on top of each. Mix cheese and mustard in a bowl and spoon a layer over the toast and soup. Bake until golden and bubbling (4-5 minutes) and serve.