Place onion slices and tomato wedges on a foil, parchment, or silpat lined baking sheet. Drizzle oil and vinegar over, and sprinkle with salt. Toss gently to combine and coat tomatoes.
Roast in 250° oven for 3 hours, stirring every hour. Once the roasting time is up, turn the oven off leaving the roasted tomatoes inside the oven to cool.
Once the tomatoes have cooled, use in recipes, or store in a tightly covered glass container in the fridge for up to 3 days, or freeze for longer term storage.
Notes
Use Roasted Tomatoes in any recipe that calls for sun-dried tomatoes