Prepare pasta according to package directions. Drain. Set aside.
Pat chicken dry with a paper towel.
Pound chicken to 1/2-inch thickness.
Dust both sides with Cajun seasoning until thoroughly coated.
In a large sauté pan over medium heat, melt 1 tablespoon butter.
Add chicken, cooking through completely, about 5 to 6 minutes for each side. Remove chicken from skillet. Slice into strips. Set aside and keep warm.
In another large sauté pan, melt 3 tablespoons butter over medium heat.
Add flour. Cook, stirring, until bubbly.
Add heavy cream. Continue to simmer, stirring, until thickened, about 6 minutes.
Remove from heat.
Add Parmesan, cayenne, salt and pepper. Stir to mix well.
Taste. Add more cayenne, salt and pepper, to taste, if needed.
Place cooked pasta in sauce. Toss to coat well.
Spoon sauced pasta onto 2 serving plates.
Top with chicken. Scatter tomatoes and green onions over top.
Serve hot with garlic toast.