Generously butter a 10-cup casserole dish or a 9x13inch baking dish.Thinly slice onions.Melt 1tbsp. butter in a heavy bottomed saucepan over med. heat. Add onions and saute for 5 minutes, stirring occasionally. Remove from pan and set aside. Melt rest of butter in pan. Blend in flour and stir constantly for 2 minutes. Gradually whisk in milk. Continue whisking over med. heat until mixture is thick and bubbling,about 5 minutes.Add salt & pepper. Crumble in half the Roquefort cheese. Stir until cheese is almost melted.Remove saucepan from heat and cover. Thinly slice green onions. Peel potatoes and slice thinly. Immediately put slices in a large bowl of cold water. Set oven temp. to 350F.Drain potatoes and pat dry. Cover bottom of casserole with 1/3 of the slices, overlapping to cover bottom completely.Scatter with half the green and red onions.Crumble half the remaining Roquefort cheese over this. Pour 1/3 cheese sauce evenly over the top. Add 1/2 remaining potatoes. Top with rest of onions and aroquefort and another 1/3 cheese sauce. Finish with rest of the potatoes and cheese sauce . Cover casserole with foil. Place on baking sheet to catch spills and bake 1 1/2 hrs.Uncover and sprinkle with Swiss cheese. Continue bakingfor30 minutes until tender. When top is golden,cover with foil. Let sit 5 min. Serves 8.