Preheat the oven to 400℉/200℃. Take the garlic head and slice ¼ off the top of the garlic head. Place the garlic on top of a piece of foil. Pull the foil up to wrap the garlic, but don’t close it yet. Drizzle 1 tbsp of olive oil. Then twist the top shut. Bake for 40 minutes until the garlic is soft and golden brown-ish.
In the meantime, slice the onions, thinly (1 inch, 2½ cm), using a sharp knife or mandolin. Heat 1 tbsp of olive oil and unsalted butter on low heat. Add the onions and stir to coat them with oil and butter. Once the onion starts to soften add the salt, sugar, and stir. Cook the onion for about 40-50 minutes, stirring occasionally, until they get dark and wrinkly (see note 1). Add the dry red wine and stir again. Transfer to a bowl.
Unwrap the garlic bulb and squeeze the garlic cloves out of it into a small shallow bowl. Using a fork, mash the garlic cloves into a paste. Add it to the onions and mix well. Put aside.
In a pan, heat the butter over medium heat. Add the all-purpose flour and whisk quickly until smooth. Add the milk and mix well. Keep whisking as the mixture starts to thicken and boil. Add the cheese and whisk again until it is all melted, incorporated, and the sauce is smooth and thick. Season with salt and pepper and transfer to a (serving) bowl.
Assemble your platter by placing the bowl with the dip on the board first. Top with plenty of caramelized onion-garlic compote. Then spread some bread first, followed by the chorizo, fuet, and lastly the Jamón Serrano. Fill the open spaces with rosemary sprigs. Serve warm!