This is so simple and so good.

6 red onions, sliced
2 teaspoons kosher salt
2 tablespoons olive oil

Preheat oven to 400 degrees. Arrange onions in a single layer in a pan. Sprinkle with salt and drizzle with oil. Roast, uncovered, for 1 hour, turning them at the ½ hour point. These can be served warm or at room temperature.