Romesco sauce is a very typical and beloved Catalan sauce that originates from Tarragona in Northeastern Spain. Roasted tomatoes combine with ground toasted hazelnuts, olive oil and vinegar to make a smooth, rich sauce that is especially delicious as an accompaniment to roasted or grilled seafood or fire roasted or grilled vegetables - but it can even be simply spread on a slice of country bread!
Preheat the grill to high heat. In a cast iron or ovenproof frying pan, toast the nuts for three to five minutes until lightly colored and toasted, being careful not to burn them or they get very bitter. Remove nuts and reserve. On the grill, lightly char the tomatoes on all sides two to three minutes each side, rotating until all sides are lightly charred. At the same time, on the grill, roast the garlic cut side down until the cut side gets nicely colored and the garlic begins to soften.
Alternatively, if using a conventional oven, place the raw hazelnuts and tomatoes (do not core the tomatoes or their liquid will spill out while roasting), and garlic, cut side down, on a baking sheet and roast in a preheated 200°C oven for about 10 minutes. Remove the toasted hazelnuts and reserve.
Return the tomatoes and garlic back to the oven for 20-30 minutes more. The tomatoes should be deeply roasted and the garlic should be nicely colored on the cut side and begin to soften.
When they are ready, remove the tomatoes and the garlic from the grill or oven. When cool enough to handle, peel the tomatoes and set aside.
In the bowl of a food processor, add the hazelnuts and the grilled or toasted bread. Lightly pulse two to three times until the bread and nuts start to break down, but are not all ground. Add the tomatoes, reconstituted Romesco or Ñora peppers, roasted garlic (squeeze the cloves out of their skin), and lightly process while gradually streaming in the oil, a little at a time, as if making a mayonnaise. Romesco should have both a fine chunky integrity to the sauce and a smooth silky texture. Finally add the vinegar and season to taste with salt and black pepper or crushed chili flakes if using. Add a little water if the sauce is too thick. The final sauce should be thick and concentrated but still loose.