Heat a small saucepan over a medium heat. Add 2 tablespoons of the olive oil, 3/4 of the chopped leeks and the garlic. Saute until softened being careful not to brown. Let cool slightly. Puree the leek mixture in a food processor until smooth and creamy. Season to taste and set aside.
Cook the pasta according to the instructions on the packet adding the outer brussel sprout leaves in the last 30 seconds of cooking.
Meanwhile, heat another small saucepan over a medium heat and add the remaining 1 tablespoon of olive oil, the leftover leeks, brussel sprouts, thyme and chilli flakes. Saute until softened, add the lemon zest, season and set aside. Drain the pasta leaving 1/4 cup of cooking liquid. Add the pureed leeks and stir through, check the seasoning. Add the sauteed leek mixture and pine nuts, toss until combined.
Divide between 4 warm bowls and sprinkle with Pecorino if using.
Alternatives:- Add some bacon to the sauteed leeks. Add flaked salmon or slow roasted cherry tomatoes.