Heat the butter and oil in a pan and fry the leek on a low heat until soft (ca 10 minutes). Add the vinegar and fiber syrup and cook another 5 minutes until the mixture caramelises.
Bring the chicken stock to a boil and cook the trimmed broccoli heads in the saucepan, lid closed, for around 4 minutes until just tender. Drain and cool.
Place broccoli on a lined baking sheet. Make 3 incisions each and stuff with the leek mixture, halloumi and sage. Drizzle with olive oil and season with salt and pepper.
Roast for around 10 minutes or until nicely browned.
Notes
If you wanted to lower the carb count, you could replace the leek with spring onions/scallions.