In a large skillet over medium low heat, heat 1 tablespoon olive oil and add the onions and salt. Sauté the onions until beginning to brown, then reduce heat to low and continue to cook, stirring occasionally, until deeply golden and caramelized, about 25 minutes total. Remove the onions to a large bowl and set aside.
Place racks in the upper and lower thirds of your oven and preheat oven to 400 degrees F. Line two large baking sheets with silicone baking mats or parchment paper. Set side.