In a large pot, bring salted water to a boil. Add the pasta and cook until very al-dente, 2 minutes less than the recommended time as it will continue to cook when you add it to the dish.  Reserve 1 cup of the cooking water, then drain pasta and set aside.

Meanwhile, heat a large skillet to medium heat, add chopped hazelnuts and dry toast until fragrant, stirring often, for about 3-4 minutes. Place hazelnuts aside in a bowl.

In the same skillet, heat 2 tablespoons olive oil until shimmering. Add ½ of the cauliflower, and ¼ tsp each salt and pepper. Cook, stirring until light golden brown and crisp, about 5 minutes.  Transfer to a medium bowl. Add another 2 tablespoons oil to the pan, heat until shimmering, and repeat searing with remaining cauliflower, remove cauliflower to bowl.

Make sure pan is still hot, and add garlic, anchovies and anchovy oil, rosemary, red pepper flakes, and 2 tablespoons butter to the pan. Cook, stirring one to two minutes until garlic is softened. It’s important to keep watch and stir so the garlic doesn’t burn, as that will give your dish a bitter flavour.

Return all the cauliflower to the pan and continue cooking and stirring for another 2-3 minutes until anchovies disintegrate. Add ½ cup pasta cooking water and bring to a boil. Add pasta and remaining 2 tablespoons butter.  Cook, tossing until the sauce has thickened and coated the pasta, about 1 minute. If it looks too dry, continue adding from the remaining ½ cup pasta water until it is to your liking. Remove from heat.

Stir in lemon juice and all but 1 tablespoon of the hazelnuts, taste and adjust salt and pepper.  Transfer to a serving platter and sprinkle with remaining nuts, parsley and lemon zest.