Heat oven to 275°. Line a baking sheet with parchment paper and drizzle with oil.
Cut tomatoes crosswise into ½-inch-thick slices. Place on prepared baking sheet and turn tomatoes to coat in oil. Arrange in a single layer. Sprinkle tomatoes evenly with sugar and season with salt.
Transfer to oven and cook until most of the moisture has evaporated and tomatoes are shriveled along the edges, 2½–3 hours. Let cool enough to handle.
Increase oven temperature to 400°. Melt butter in a 10-inch cast iron skillet over medium-low heat. Add onion, season with salt and pepper, and cook, stirring often, until golden brown and sweet, about 12 minutes.
Meanwhile, whisk together eggs and half and half. Pour over caramelized onions in skillet, sprinkle evenly with cheese and let cook until edges are set and bubbling, about 5 minutes. Arrange slow roasted tomatoes decoratively on top and transfer to oven. Cook until center is set and edges are beginning to brown, about 10 minutes.