Cook the pasta according to the package directions. Once cooked, reserve 1 cup of pasta water and drain.
In the same pot you cooked the pasta in or a large skillet, heat the olive oil and melt the butter over medium-high heat. Add the onions and saute until soft and translucent, for about 3 minutes. Add the garlic and creole seasoning, stir, and continue cooking for 2 more minutes.
Add the half-and-half and parmesan cheese and mix until combined. Bring the mixture to a simmer and then reduce the heat to medium-low and simmer the sauce for 3 minutes. Then, add the crawfish tails to the sauce and simmer for an additional 3 minutes.
Add the pasta and half of the reserved pasta water and mix them with the creamy crawfish sauce. Slowly add the remaining pasta water to make it more saucy.
Serve crawfish pasta topped with parmesan cheese.
Notes
If using leftover crawfish tails from a boil that are already seasoned, I suggest using slightly less of the creole seasoning.