Melt butter over medium heat in a large pot or dutch oven. Add onions and cook, stirring occasionally, until onions are golden and caramelized. Take your time - this may take 20-30 minutes. Add garlic and continue to cook for another few minutes.
Meanwhile, boil the pasta according to the package directions. Retain about one cup of the pasta water before you drain the cooked pasta. Toss hot pasta with a bit of oil.
Add chopped kale to the onions, stirring until kale is tender, 3-5 minutes. Season generously with salt and freshly ground black pepper.
Add cooked pasta to the vegetables. Use tongs to toss the mixture and combine. Add olive oil, lemon zest and juice, and ricotta. While combining the mixture, add reserved pasta water a little at a time as needed to create a smooth, silky consistency.
Variation: this recipe is a delicious vegetarian meal, but it also pairs well with thin-sliced chicken breast, sausages, ground turkey or ground chicken. After onions are caramelized, sauté meats in the dutch oven until fully cooked, then proceed with the remainder of the recipe.