Heat olive oil in a casserole over medium heat, add pancetta and cook until starting to crisp (2-3 minutes). Add garlic and sage and cook until just starting to colour (1 minute). Add Brussels sprouts, stir, then add chicken stock and butter and cook over medium heat until soft (14-15 minutes). Serve immediately as an accompaniment to fish or chicken.
Note Single-clove garlic is milder and sweeter than common garlic. If unavailable, substitute with 2 cloves of garlic per single-clove garlic.