Cook the Spaghetti: Bring a large pot of salted water to a boil. Add the spaghetti and cook for about 10 minutes, until just shy of al dente. Reserve ½ cup of pasta water, then drain the pasta. Toss the cooked spaghetti with 1 tablespoon of olive oil and set aside.

Prepare the Baking Dish: Preheat your oven to 350°F (175°C). Grease a 13×9-inch baking dish with nonstick spray and set aside.

Sauté Vegetables: In a large dutch oven, heat the remaining tablespoon of olive oil over medium-high heat. Add the diced onion, bell pepper, salt, and pepper. Sauté for 5 minutes, or until the onion becomes translucent and soft.

Cook with Tomatoes: Add the diced tomatoes with green chilies. Cook for about 5 minutes, or until most of the liquid has evaporated.

Make the Sauce: Stir in the cream of mushroom soup, cream of chicken soup, and chicken stock. Bring the mixture to a simmer.

Melt the Cheese: Add the cheddar cheese and Velveeta cubes, stirring until completely melted and the sauce is creamy. Add the cooked chicken, and season with salt and pepper to taste.

Combine Pasta and Sauce: Gently fold the cooked spaghetti into the sauce, using some reserved pasta water if needed to achieve the desired consistency.

Transfer and Bake: Pour the mixture into the prepared baking dish. Bake for 30-40 minutes, or until the top is golden and the sauce is bubbly. Serve hot.