1 large green bell pepper, cut into bite-sized pieces
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1 large red bell pepper, cut into bite-sized pieces
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1 large onion, chopped
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4 cloves garlic, minced, divided
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6 medium Roma tomatoes, diced
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2 tablespoons butter
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1 tablespoon all-purpose flour, or more as needed
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1 ½ cups whole milk, or more to taste
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½ cup grated Parmesan cheese, or more to taste
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salt and ground black pepper to taste
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1 ½ cups shredded mozzarella cheese
Instructions
Preheat the oven to 400 degrees F (200 degrees C).
Bring a large pot of lightly salted water to a boil. Add ziti and cook, stirring occasionally, until tender yet firm to the bite, about 10 minutes. Drain.
While the pasta is cooking, heat oil in a large skillet over medium to medium-high heat. Add chicken, Cajun seasoning, and salt. Saute until chicken is cooked through and beginning to caramelize, about 10 minutes. Add bell peppers, onion, and 1/2 of the garlic; cook until tender, 5 to 7 minutes. Add tomatoes and bring to a simmer.
Meanwhile, melt butter in a saucepan over medium heat. Add remaining garlic and saute until fragrant, about 1 minute. Add flour; cook and stir for 1 minute. Slowly whisk in 1 1/2 cups milk and bring to a gentle boil. Stir in 1/2 cup Parmesan cheese, salt, and pepper. Add more Parmesan to thicken or more milk to thin to your liking.
Add cooked pasta to the chicken mixture; toss to coat. Transfer to a deep casserole dish. Pour cheese sauce over the pasta mixture and top with mozzarella cheese.
Bake in the preheated oven until the top is golden and bubbly, about 15 minutes.