Over an open flame (like a grill) or the grates of your gas stove, place the red peppers atop, rotating them every minute or so until charred on all sides. Repeat this with the tomato. Immediately transfer them to a Ziplock-bag and close; this will allow them to steam for 1 to 2 minutes. Transfer them to a cutting board and scrape the skins off using a knife. Discard the burnt skins. Chop up the red bell peppers, discarding the seeds and stem, and tomato and transfer to a blender or food processor, along with the garlic, almonds, hazelnuts, olive oil, sherry vinegar, paprika and salt.
In a sauté pan, set over medium heat, add a few tablespoons of butter. When the butter is melted, add the leeks, cut side down, and cook for about 2 to 3 minutes, until golden brown. Flip and cook on opposite side for an additional 2 to 3 minutes. Transfer to oven and braise for about 15 minutes. Serve room temperature with a few tablespoons of romesco spooned on top.
You won't use the whole batch on the leeks so be sure to save the rest in an air-tight container. Keeps well in the fridge for up to two weeks.