Preheat a large grill pan or griddle over medium to medium high heat. Preheat a small skillet over medium to medium high heat. Add 1 tablespoon of oil and butter to the small skillet. Add onion to the melted butter and oil. Saute the onion, stirring frequently, 10 to 12 minutes or until onions are caramel colored and soft. Build sandwiches: spread 1/4 of the onions on each of 4 slices of bread. Top onions with a thin layer of fontina and sprinkle with chopped sage. Add a thin layer of pears to each sandwich, then more fontina and the top slice of bread. Drizzle the sandwiches with extra-virgin olive oil. Place drizzled side down and arrange the sandwiches on hot grill or griddle. Drizzle opposite side of each sandwich with oil and weigh the sandwiches with foil wrapped bricks or with a heavy skillet, weighted with a sack of flour or canned goods. Press sandwiches 2 or 3 minutes on each side and serve immediately.