Melt butter in a Dutch oven over medium. Add onion and ½ teaspoon salt; cook, stirring occasionally, until softened, about 6 minutes. Add garlic and pepper; cook, stirring constantly, about 1 minute.
Add chicken stock and 2 ½ teaspoons salt to mixture in pot; bring to a boil over high. Add pasta; cook, stirring often, until pasta is al dente and liquid has thickened, about 14 minutes.
Remove pot from heat; stir in cream cheese and Parmesan until melted. Stir in chicken and diced tomatoes. Garnish with scallions and more Parmesan and black pepper.
