Preheat the oven to 275 degrees. Line a baking sheet with parchment or foil, then coat it with cooking spray.
Core the tomatoes. Slice the larger tomatoes in half and leave the small ones whole.
Place on the baking sheet, cut side up.
Slide the slivers of garlic into the tomato halves. Season with sea salt, black pepper, basil, and oregano to taste. Drizzle the top of the tomatoes with the olive oil.
Place in the oven and roast for 3½ hours, rotating the tray halfway through roasting.
Place on the counter to cool to room temperature.
Once they are cool, put the tomatoes and their juices in a storage bowl covered with an airtight lid. Refrigerate until needed.