In a measuring cup or small bowl, mix the sour cream, mayonnaise, ranch dressing mix, and 3 tbsp water until combined.
Heat a large deep skillet over medium-high heat and add the olive oil. Sautee the chopped onion and minced garlic until translucent, about 2-3 minutes. Mix in the softened cream cheese until melted.
Pour in the sour cream mixture, chicken broth, and buffalo sauce, and mix until combined.
Add the shredded mozzarella and cheddar cheese and stir until melted.
Mix in the cooked and drained pasta, diced tomatoes, and shredded chicken until fully combined. Cook for 2 minutes or until chicken and pasta are heated through. Check the seasoning, and add salt and pepper if needed. Serve with sliced green onions on top.