Romesco is one of Spain's classic sauces, typically made with bread and charred vegetables. I like to lighten it up by taking the bread out, adding more almonds, and swapping red peppers for carrots to give it an earthy dynamic. It's the perfect sauce to go along with just about anything, from grilled vegetables to fish.
Roast the vegetables: Heat oven to 550 degrees F, or as high as your oven will go. Leaving the skin on, cut the onion in half vertically and place in roasting pan, cut side down. Cut the carrots in half horizontally and add to pan. Slice tomatoes in half and add to pan, cut side down. Cut head of garlic in half horizontally, discard the woody outermost layer, and add to pan, cut side down. Drizzle ¼ cup olive oil over the vegetables and roast until soft and lightly charred, about 15 minutes.
Make the romesco: When the vegetables are roasted and cooled, remove the onion, tomato, and garlic skins and discard; add all the vegetables to the food processor. Deglaze the roasting pan by adding enough water to cover the bottom, heating to a low simmer, and scraping to release the brown bits (also known as "fond"). Meanwhile, pulse the vegetables in the food processor to a chunky mash. Add the sliced almonds, toasted almonds, salt, pimenton, vinegar, and 3 tablespoons of olive oil; pulse to combine.
Finish the romesco: Add half of the deglazed pan juices to the food processor and continue to pulse to combine; then season to taste with more salt. With the motor running, add the remaining tablespoon of olive oil to finish. Serve with grilled vegetables, as a dip, or as a vinaigrette by mixing a few tablespoons of romesco with one part vinegar and two parts oil.