Preheat the oven to 325 degrees Fahrenheit. Slice the tomatoes in half and place them cut side down in a roasting dish. All of the tomatoes must be touching the bottom of the pan; if you layer them, they won't cook properly, so make a smaller quantity of sauce or divide into two pans if needed.
Scatter the peeled garlic cloves on top of the tomatoes, and drizzle 3 tbsp of olive oil over everything. Sprinkle with the salt (and pepper, if desired). Roast until the tomatoes are tender, their juices released into the pan and the skins wrinkled, and the garlic cloves are also tender, about 80-90 minutes. After 45-60 minutes, you can gently stir. I kind of push the tomatoes around with a spatula, rather than outright stirring them, so that they stay cut side down.
Transfer the roasting pan mixture to a blender or food processor, and blend until smooth. Season to taste with additional salt; I use plenty. Serve warm, with pasta or accompaniment of choice.