Make onion soubise.
Heat a medium saucepan over low heat. Add the butter, ½ cup of water and the onions to the pan. Continue to simmer the onions over low heat, stirring occasionally, until they are tender, about 1½ hours.
In a large pot over medium heat add the olive oil and heat through. Add the garlic and sauté until fragrant, just a few seconds.
Simmer tomato sauce.
Add the tomato paste, the tomatoes and sugar, stirring to combine. Let simmer for 15 minutes, breaking up any large pieces of tomato. Season to taste with salt.
Add onion, cream & chilies.
Add the onion mixture to the pot and stir to combine then whisk in the cream, chili paste and chili flakes.
Cook the pasta.
Bring a large pot of water to a boil; add the salt and bring back to the boil. Add the pasta and cook, stirring occasionally, until al dente, about 8 minutes. Reserve 1 cup of the pasta water then drain, and return pasta to the pot.
Assemble pasta and sauce.
Ladle the sauce over the top and toss to combine, cooking for an additional minute. If the sauce is too thick, add the pasta water 2 tablespoons at a time to thin the sauce.
Serve the spicy rigatoni hot in warmed pasta bowls with grated Parmesan cheese (optional).