To make meat sauce: Heat olive oil in large frying pan. Saute onion until golden. Add ground beef, seasonings, tomato sauce and cover. Simmer for 30 minutes or transfer to crock pot and cook on low for 8 hours. Mix in 1 cup sliced pitted olives if desired. Place a square of foil in bottom of crock pot. Lighly butter one side of tortillas. Lay one tortilla, buttered side up, on foil. Spread with layer of meat sauce and layer of cheese. Cover with another tortilla, more meat sauce and cheese. Repeat layers ending with cheese. Cover and cook on high for 1 hour. When ready to serve, lift from crockpot by foil corners. If desired slip under broiler to brown cheese. Cut into wedges. Combine sour cream and onions. Serve with Tostada wedges.