The bulb-shaped stem base has a distinct aniseed flavour that becomes milder during cooking. Larger bulbs have tough cone-shaped cores, so cut these out. You can use the edible fronds for garnish and keep the stalks for stocks.
Heat half the butter in a widebased saucepan until foaming. Add the fennel and onion, and sweat over a low-medium heat for about 1 hr, adding little slices of butter if needed.
After 1 hr, increase the heat and keep on cooking down for another 10 mins, stirring occasionally, until caramelised and jammy, and beginning to brown.
Put the remaining butter in a separate pan with a splash of vegetable oil and fry the bacon until crisp. Add to the fennel compote along with the parsley, and season to taste. Will keep for 3 days in an airtight container.