Slice the tomatoes so they are about 1/4 inch thick. I find the meatier they are the better they turn out.
On a parchment-lined baking sheet, spread the tomatoes out in a single layer and drizzle olive oil over them. Then sprinkle them with the chopped rosemary. Add the cloves of garlic to the sheet, leaving them peeled and in tact.
Roast the tomatoes for 6 hours at 200 degrees F. The meat of the tomato should shrivel but they will still be a bit liquidy.