Preheat the oven to 275°F.
Line a sheet pan with foil or parchment paper.
In a bowl, toss the tomatoes with olive oil and season with salt and pepper.
Arrange them skin-side down on the lined sheet pan, tucking thyme sprigs (if using) in between the tomatoes.
Bake until they are shriveled and deep red, about 1.5 hours.
Let cool, then refrigerate in an airtight container for up to 5 days.
