Preheat your oven to 250°F (120°C). Line two large baking sheets with parchment paper.
Arrange the halved Roma tomatoes cut-side up on the prepared baking sheets. Try not to let them touch too much.
In a small bowl, whisk together the olive oil, dried oregano, dried basil, garlic powder, sea salt, and black pepper. Drizzle this mixture evenly over the tomatoes.
If using, tuck the thinly sliced garlic cloves in between the tomatoes on the baking sheets.
Place the baking sheets in the preheated oven. Bake for 4-6 hours, or until the tomatoes are shriveled, leathery, and have a concentrated flavor. The exact time will depend on the size and juiciness of your tomatoes and your oven. Check them periodically to ensure they aren't burning.
Once dried to your desired consistency, remove the baking sheets from the oven and let the tomatoes cool completely on the sheets.
Once cooled, you can store them as is, or place them in a jar and cover them with good quality olive oil for a more intense flavor and longer shelf life. If storing in oil, ensure the tomatoes are fully submerged.