1. Preheat broiler to high.
2. Cook pasta according to package directions, omitting salt and fat; drain.
3. While pasta cooks, combine milk, stock, flour, and salt in a medium saucepan, stirring with a whisk. Bring to a boil over medium heat; cook 1 minute or until thickened. Let stand 5 minutes. Add 2 ounces shredded cheddar and processed cheese, stirring until smooth.
4. Heat a 10-inch cast-iron skillet over medium-high heat. Add oil; swirl to coat. Add onion, oregano, thyme, pepper, and garlic; cook 3 minutes, stirring occasionally. Combine onion mixture, parsley stems, cilantro stems, and lime juice in a food processor; pulse until coarsely chopped.
5. Combine pasta, milk mixture, and parsley mixture. Heat cast-iron skillet over high heat. Coat pan with cooking spray. Add pasta mixture to pan; sprinkle with remaining 1/2 ounce cheddar cheese. Broil 8 inches from heat for 5 minutes or until cheese is browned. Sprinkle with cilantro leaves.