In a saucepan, melt 2 tablespoons butter or margerine and cook the onions for 5 minutes, or until soft. Do not brown. In a large bowl, combine the onions and all the remaining, except the potatoes, the remaining butter or margerine, and olive oil, mixing well.

Spread out and oil a large piece of wax paper. Reserve 1/2 cup of the meat mixture. Spread the remaining meat mixture on the wax paper, making a 10-by-7-by-1/2-inch rectangle. Mold the meat into a solid cake. Spread the mashed potatoes on top, smoothing tem evenly with the flat of a knife.

Lifting up the wax paper, roll the mixture jelly-roll fashion as tightly as possible, to completely encase the potatoes. If there are any holes, patch with the reserved meat. If not, pat the reserved meat onto the ends of the roll.

Carefully slide the roll off the paper onto a shallow, lightly buttered baking pan. Bake, uncovered, in a preheated 350° F oven for 1 hour.

Heat the remaining 2 tablespoons butter or margerine with the olive oil and baste the meat-potato roll with it.