Start by prepping the onions - cut the onions in half, then cut the halves into thin, C-shaped slices.
While onions are cooking down, cook the pasta according to package directions. I used casarecce but you can use any sort of tubular pasta, like ziti or penne. Rotini or fusilli would probably work too. When pasta is done cooking, drain and set aside in the pot you used to cook it in.
When onions are done cooking, they should be a gorgeous brown color and will smell amazing. Scrape the onions into the pot full of pasta, and mix well. Add in juice from the lemons (taste and adjust if needed), and salt and pepper to taste.