Bring a medium saucepan of water to a boil. Add the cipolline and blanch for 60 seconds exactly.

Transfer immediately to a bowl of ice water with a slotted spoon. Let them sit for 2 minutes until cool enough to handle.

Trim the root end of each onion with a paring knife and squeeze gently from the stem end - the skin should slip off cleanly. Pat dry.

Place a wide 12-inch skillet over medium heat. Add the butter and olive oil and let the butter melt until foamy.

Add the sugar and stir for about 30 seconds until it dissolves into the fat and just begins to turn a pale gold at the edges.

Add the peeled cipolline in a single layer. Turn them gently to coat in the butter-sugar mixture. Add thyme and bay leaf if using.

Pour in the red wine vinegar - it will sizzle. Stir to combine, scraping up any bits from the pan.

Add the water or broth and the salt. Bring to a gentle simmer, then reduce heat to medium-low.

Braise uncovered for 18 to 22 minutes, turning the onions every 5 minutes, until they are tender when pierced with a knife and the liquid has reduced to a thin, glossy glaze that clings to them.

Remove the thyme and bay leaf. Taste and adjust salt. Serve warm or at room temperature.