Preheat oven to 400° F. In skillet, place sausages and 1/4 cup water. Bring to simmer over medium heat; cover and cook 10 minutes. Uncover; discard liquid and cook 15 minutes, turning until cooked through. Drain on paper towels.
Heat butter in same skillet over medium heat, add onions and cook, stirring occasionally, 20 minutes. Add scallions, vinegar, olives and 2 tablespoons sage; cook 8 minutes. Meanwhile, prepare pasta as package directs.
Cool slightly and toss with eggs, cheese, remaining 2 tablespoons sage,half and half and pepper. Place in greased 2-quart casserole. Bake 15-20 minutes until pasta shell is set and lightly browned.
Slice sausages; add to onions and season with salt and pepper to taste. Spoon into pasta shell.
Servings: