In a medium saucepan, heat the butter and oil over medium heat. Sprinkle in the flour, and using a whisk, stir to create a paste. Cook for 1 to 2 minutes. Add the milk and cream bit by bit, whisking between each addition. Let the sauce cook and thicken, about 5 minutes. Add half the basil, rosemary, parsley, and scallions and all the sun-dried tomatoes. When the sauce is thick enough to coat the back of a spoon, remove from the heat and whisk in half the cheese. Taste for seasoning, adding more salt and pepper as needed.