Choose a saucepan just large enough to loosely hold all the fennel in a single layer. Don’t add the fennel yet, instead heat the olive oil over medium high heat.
Add the garlic and saute until fragrant, about 1 minute.
Place the fennel pieces cut side down in the pan in a loose single layer. Cook until just beginning to color then turn each piece over.
Add just enough water to cover them about halfway.
Add butter, sugar, pepper flakes & a pinch of salt.Bring the water to a boil, then lower the heat to very low. Simmer about 15 minutes until the fennel is tender and the sauce has been reduced to a syrupy glaze, then add vinegar. If the vegetables seem done before the glaze is achieved gently remove them to a plate using a slotted soon, then return them to the pan to gently reheat once the liquid has reduced to the desired consistency.Season the vegetables with the black pepper and spoon the glaze over them to coat thoroughly.