Step 1: Heat olive oil in a large skillet or Dutch oven over medium-high heat. Season chicken cubes with 1 tablespoon of Cajun seasoning. Add chicken to the hot skillet and cook until browned on all sides and cooked through, about 5-7 minutes. Remove chicken from the skillet and set aside.
Step 2: Reduce heat to medium. Add butter to the skillet. Add chopped onion and cook until softened, about 3-5 minutes. Add minced garlic and cook for another minute until fragrant.
Step 3: Pour in chicken broth, scraping any browned bits from the bottom of the pan. Bring to a simmer, then stir in heavy cream.
Step 4: Reduce heat to low. Add the cubed Velveeta, shredded mozzarella, and grated Parmesan cheeses. Stir continuously until all cheeses are completely melted and the sauce is smooth and creamy. Stir in the remaining 1 tablespoon of Cajun seasoning. Season with salt and pepper to taste.
Step 5: While the sauce is simmering, cook the spaghetti according to package directions until al dente. Drain well, reserving about 1/2 cup of pasta water.
Step 6: Add the cooked chicken and drained spaghetti to the creamy cheese sauce. Toss gently to coat everything evenly. If the sauce is too thick, add a little reserved pasta water, one tablespoon at a time, until desired consistency is reached. Garnish with fresh chopped parsley, if desired, and serve immediately.