Peel and cut the potatoes into 1 inch chunks. Boil the potatoes for 10-15 minutes until done. Drain the potatoes and place them in a bowl. Mash the potatoes with 2 tablespoons of butter and the chicken stock. Set aside.
Slice the onion in half and then cut into 1/4 inch slices. Saute the onions in the 2 tablespoons of olive oil until translucent, next add the garlic and sage.
Remove the sausage from its casing and add to the pan with the onions and galic and cook till the suasage is browned and no longer pink.
Add the sausage mixture to the potatoes and mix till combined. Add the salt and pepper. You may add additional salt and pepper to your taste.
Place the mixture in a buttered casserole. At this point you may refrigerate overnight or you can place in preheated 350 degree over for 20-30 minutes or until heated through.