Preheat oven to 350°F Butter 8x8x2-inch glass baking dish.
Cook sausage in heavy large skillet over medium-high heat until brown, breaking into small pieces with back of spoon, about 5 minutes. Mix in flour, then milk. Cook until mixture thickens and comes to boil, stirring occasionally, about 5 minutes.
Layer potatoes in prepared dish. Top with 1/3 of green onions, 1 cup cheese, 1/3 of green onions, sausage mixture, sprinkle the herbs on top, and the remaining 1/4 cup cheese. Bake casserole until potatoes are tender, about 45 minutes. Sprinkle with remaining green onions and serve.