Coarse kosher salt and freshly ground black pepper
Heat olive oil in a large cast iron or stainless steel skillet over medium heat.
Cut the ends off the onions, remove outer skin, cut in half, and then slice thinly from end to end. Add onions and garlic to the oil and stir to coat in oil. Reduce the heat to medium low and allow the onions and garlic to cook, barely sizzling and stirring every 5-10 minutes. The onions and garlic are done when they are soft and golden, 40-60 minutes. Do not allow the garlic to burn. If things are browning too quickly reduce the heat slightly.
Meanwhile, bring a large pot of water to a boil and cook rigatoni according to package directions. Just before you drain the pasta, reserve ½ cup of the pasta water. Drain the pasta and return it to the pot.
Once the onions are completely caramelized, add ¼ cup of the reserved pasta water to the pan, scraping up all the browned bits on the bottom of the pan. Stir until the onions have almost completely absorbed the water and add the gorgonzola cheese. Stir for about a minute or until the cheese just begins to melt. If the mixture looks dry at all you can add a bit more pasta water. Transfer the entire mixture to the pot with the pasta making sure to get all the oil and any browned bits off the bottom of the pan. Stir thoroughly to coat the pasta with the cheese, onions, and garlic. Season to taste with salt and pepper.