Bring a pot of salted water to a boil. Add the pasta and cook until al dente, about 1 minute less than the label directs. Reserve 1 cup of the cooking water, then drain the pasta. Meanwhile, combine the olive oil and sausage in a large skillet over high heat and cook, stirring, until the sausage starts to brown, about 2 minutes. Stir in the scallions and bell pepper and cook until the vegetables are slightly soft, about 3 more minutes. Add the Cajun spice and flour and cook, stirring, 2 more minutes. Gradually whisk in the milk and continue whisking until the sauce is thick and smooth, about 5 more minutes. Add the pasta, cheese and reserved cooking water to the skillet. Reduce the heat to medium low and cook, tossing to coat the pasta in the sauce and melt the cheese, about 1 minute.