Make the marinade. In a small bowl, combine the olive oil, garlic and rosemary. Set aside.
Preheat the oven and prepare the sheet pan. Preheat the oven to 225°F, drizzle half of the marinade on a sheet pan and sprinkle it with salt and pepper.
Peel, cut and seed the tomatoes. Bring a large pot of water to a boil and set up a large bowl of ice water. Core the tomatoes and slice an "X" on the bottom of each one -- trying only to cut through the skin and not into the tomato. A few at a time, add the tomatoes to the boiling water and leave them only long enough for the skin to begin to pull away from the "X," about 30 seconds. Use a slotted spoon to add them to the ice water. When they're cool enough to handle, remove them from the water and gently peel the skin off. Cut each peeled tomato in half, horizontally, and use your fingers or a small teaspoon to scoop out the seeds. Gently shake the tomato half to remove excess seeds.
Prepare the tomatoes to roast. Place the tomatoes on the prepared sheet pan, round side up. Sprinkle with salt and pepper and drizzle the remaining half of the marinade over them. If they're not well coated, use a spoon to drizzle it in from the pan. (Don't worry if it's a tight fit -- squeeze in all of the tomatoes, as they will shrink during the cooking process. )
Slow roast. Roast the tomatoes in the preheated 225°F oven for approximately 5 hours, basting every hour or so with the pan juices. When the tomatoes are done, they will be about half their original size and slightly golden along some of the edges. They should not be dry. Season to taste with salt and pepper. (Here's How to Season to Taste.)