Bring a large pot of water to a boil. Salt it generously, it should taste like mild seawater. Cook the rigatoni according to the package directions until just al dente. Before draining, scoop out at least 1 cup of pasta water and set it aside. Drain the pasta and set it aside too.

While the pasta cooks, heat the sun-dried tomato oil in a large skillet over medium heat. Add the garlic and red pepper flakes if using, and cook for about 1 minute, stirring, until the garlic smells fragrant but has not browned.

Add the chopped sun-dried tomatoes to the pan and stir them around for another minute so they heat through and release their flavor into the oil.

Pour in the heavy cream and stir to combine. Let the sauce simmer on medium-low heat for 3 to 4 minutes, stirring occasionally, until it thickens slightly and coats the back of a spoon. You will see it go from thin and pale to a soft blush color.

Add the parmesan cheese a little at a time, stirring after each addition until it melts in smoothly. Season with the salt and pepper and taste. Adjust as needed.

Add the baby spinach to the sauce and stir gently for about 1 minute until it wilts down. It will look like a lot at first but shrinks quickly.

Add the drained pasta to the skillet and toss well to coat everything in the sauce. If the sauce looks too thick or the pasta is clumping, splash in pasta water a few tablespoons at a time and toss until it is silky and loose. Serve immediately with extra parmesan on top.