In a large pan, melt 2 tablespoons of butter over medium heat. Add the sliced leeks and onions, cooking them slowly on low heat until they are golden and sweet, which should take about 20 minutes.

Meanwhile, bring a pot of salted water to a boil and add your pasta. Cook according to package instructions until al dente, usually about 8-10 minutes, reserving a cup of pasta water before draining.

Add the remaining tablespoon of butter to the same pan with the caramelized leeks. Toss in the finely chopped garlic, cooking until fragrant, about 1-2 minutes. Stir in the cream and grated parmesan, whisking until the sauce is smooth.

Return the caramelized leeks and onions to the creamy sauce. Add in the lemon juice, rosemary, salt, and pepper, stirring to combine. Gently toss in the cooked pasta.

Transfer the creamy pasta to plates and garnish with extra parmesan cheese if desired. Serve immediately while hot.