4ounceslight cream cheesesoftened to room temperature
Bring a large pot of salted water to a boil. Prepare pasta according to the package instructions for al dente.
Heat the oil in a large, deep skillet or dutch oven over medium-high heat. Place the shrimp in a bowl and sprinkle with 1 teaspoon Cajun seasoning, 1/4 teaspoon garlic powder, 1/4 teaspoon kosher salt, and 1/8 teaspoon black pepper. Toss to coat.
Once the oil is hot but not yet smoking, add the shrimp.
Cook just until they turn from gray to pink, are lightly firm, and fully cooked through, about 3 to 5 minutes. (Be careful not to overcook!) With a slotted spoon, immediately remove them to a large plate or large bowl. Reduce the skillet heat to medium.
Heat 1 tablespoon butter over medium heat. Add the red, orange, and green bell peppers. Cook 3 minutes, then add the mushrooms and sprinkle with 1 teaspoon Cajun seasoning and the remaining 1/4 teaspoon garlic powder, 1/2 teaspoon kosher salt, and 1/8 teaspoon black pepper. Sauté until the mushrooms are tender and have given off their liquid, about 5 additional minutes. Add the tomatoes and cook until most of their liquid has cooked off, about 3 minutes.
Add two-thirds of the green onions. With a large spoon, scoop up the vegetables into the plate or bowl with the shrimp.
In a large liquid measuring cup, whisk together the milk and cornstarch until well combined.
Grab a whisk and heat the final 1 tablespoon butter over medium. Pour in the milk mixture and whisk until smooth. Add the cream cheese in small pieces. (I usually tear and drop it right in with my fingers.) Whisk constantly and bring the mixture to a steady (but not violent) simmer, and cook for 2 to 3 minutes until the sauce is slightly thickened, whisking all the while so that you have a smooth, creamy sauce. Stir in the final 1 teaspoon Cajun seasoning.
Return the vegetables and shrimp to the skillet. If there are a lot of juices that have collected in the bottom of the bowl, you can choose to add it if you want a thinner sauce or omit it if you prefer a thicker sauce. Drain the noodles and add them to the skillet.
With a long pair of tongs, toss to fully combine and coat every bit with that yummy, creamy sauce. Taste and adjust salt, pepper, and Cajun seasoning to taste. Sprinkle with remaining green onions. Serve hot.
*For the speediest possible prep, purchase peeled and deveined shrimp. (Easy-peel and deveined would also work.) I buy frozen and let them thaw overnight in the refrigerator. Tails on or off is your preference.
TO STORE: Refrigerate pasta in an airtight storage container for up to 3 days.
TO REHEAT: Gently rewarm leftovers in a Dutch oven on the stovetop over medium-low heat or in the microwave.
TO FREEZE: Freeze pasta in an airtight, freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.