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Heat 2-3 tablespoons of olive oil in a skillet over low heat. Cook the onions until deep golden, about 15-20 minutes, stirring occasionally.
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Prepare the tomatoes by mixing them with dried oregano, salt, and 1 tablespoon of olive oil in a bowl. Set aside.
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Combine yogurt, all but 1 teaspoon of garlic, a pinch of salt, 2 tablespoons of olive oil, and lemon zest in a bowl. Stir well.
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Mix all fresh herbs together and set aside.
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Coarsely grind walnuts in a food processor and toast them in a dry skillet with half a teaspoon of garlic and a pinch of salt.
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Cook pasta in boiling salted water, reserving 2 cups of cooking liquid. Drain and toss pasta with 2 tablespoons of olive oil.
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Assemble the dish by placing pasta in a large bowl, topping with yogurt mixture, caramelized onions, herbs, and walnuts. Toss everything together and serve with marinated tomatoes scattered around the plate.