Caramelize the leeks & onions: In a large skillet, melt 2 tbsp butter over medium heat. Add the sliced leeks and onions, cooking them slowly on low heat until they are softened, about 15-20 min. Crank heat up to medium and add white wine. Once that cooks off, turn off heat while you make the rest of the pasta.
Cook the pasta: Meanwhile, bring a pot of salted water to a boil and add in pasta. Cook according to package instructions until al dente, reserving a cup of pasta water.
Make the sauce: Melt the remaining tbsp of butter to a deep skillet (big enough that you'll be able to add the pasta to at the end). Add minced garlic, cooking until fragrant, about 1-2 minutes. Stir in the cream, grated parmesan, and a nice pinch of salt and pepper. Whisk until it is smooth and has started to thicken. If it becomes too thick, adjust with some reserved pasta water until its the creaminess you want.
Add pasta to the skillet with the sauce and then top with the caramelized leeks, capers and lemon juice. Mix until super creamy and combined. Top with more grated parmesan. Enjoy!