Adjust oven rack to middle position and heat to 400°F Lightly butter a 9 1/2-inch deep dish pie plate.
Place potatoes in large pot and add enough water to cover by 1 inch. Add salt to taste. Bring potatoes to boil over high heat, then lower heat to maintain a gentle simmer. Cook until potatoes are barely tender but still firm, about 7 minutes. Be careful not to overcook potatoes; they should still hold their shape nicely. Drain.
Meanwhile, fry bacon in large skillet over medium heat until crisp, about 10 minutes. Use slotted spoon to transfer bacon to plate lined with paper towels and drain well. Discard all but 2 tablespoons bacon fat in pan. Add onions to skillet and cook over medium-high heat, stirring occasionally, until onions wilt and begin to brown, about 7 minutes. Reduce heat to low and continue to cook until onions are very soft and brown, 25 to 30 minutes. Stir in garlic and remove pan from heat.
Cover bottom of prepared pie plate with single layer of potato slices and then sprinkle with salt and pepper to taste. Spread half of the onions over potatoes, followed by 1/3 of the bacon and 1/3 of the cheese. Arrange second layer in same way, seasoning with salt and pepper and topping with remaining onions and half of remaining bacon and cheese. Cover with remaining potatoes, arranging slices slightly overlapping around outside of pan and then filling in open spaces with remaining potato slices. Sprinkle with bits of butter, remaining bacon, and salt and pepper to taste. Pour broth over casserole and sprinkle with remaining cheese.
Cover pie plate with foil and bake for 30 minutes. Uncover and continue to bake until cheese has melted and top is golden brown, about 15 minutes.