Fill a large pot with water, bring to a boil, and cook the pasta according to package instructions until al dente. Drain and set aside.
Meanwhile, heat vegetable oil in a large skillet over medium heat. Add sliced Andouille sausage and cook, stirring occasionally, until browned and slightly crispy around edges, about 5-7 minutes.
Add sliced bell peppers and onions to the skillet with the sausage. Sauté until vegetables soften and develop some charred spots, about 3-4 minutes. Add minced garlic and cook until fragrant, about 30 seconds.
Sprinkle Cajun seasoning, salt, and black pepper over the mixture. Stir well to coat all ingredients evenly and cook for another 1-2 minutes to allow flavors to meld.
Add the cooked pasta to the skillet and toss everything together until well combined and heated through, about 2 minutes. The pasta should absorb some of the smoky, spicy flavors from the sausage and seasoning.
Transfer to serving plates, optionally garnish with chopped green onions or hot sauce, and serve immediately while hot.