Place a rack in the center of your oven and heat to 375°F. Coat a 9x13-inch baking dish with nonstick spray.

Bring a large pot of salted water to a boil. Cook the pasta to al dente according to the package instructions. Drain and return to the pot. Drizzle with olive oil and toss to prevent sticking.

Meanwhile, in a large skillet, heat 1 tablespoon olive oil over medium heat. Add the onion and bell pepper and cook, stirring, until beginning to soften, about 5 minutes. Add the garlic and ½ teaspoon salt and cook for 1 minute. Transfer the vegetable mixture to the prepared baking dish.

Return the skillet to medium-low heat and add the broth, milk, and cream cheese. Whisk until the sauce is hot and the cream cheese is melted, about 3 minutes. Add the undrained can of tomatoes, pimentos, ½ cup cheddar, mozzarella, paprika, cumin, and remaining ½ teaspoon salt. Bring to a low simmer and cook, stirring occasionally, until the sauce has thickened, 5 to 7 minutes.

Add the cooked spaghetti, chicken, and cheese sauce to the baking dish with the vegetables. Stir to combine. Top with the remaining 1 cup shredded cheddar.

Bake uncovered until bubbling and golden on top, 20 to 25 minutes. Let cool for 5 minutes before serving.