Preheat your oven to broil. Prepare a large baking sheet and arrange the halved Roma tomatoes, sliced onion, and whole garlic cloves on it. Drizzle with olive oil and sprinkle with coarse sea salt and black pepper.

Place the baking sheet under the broiler and roast for about 15 minutes, or until the tomatoes are blistered and the onions are soft and translucent. Keep an eye on them to prevent burning.

Once roasted, remove the baking sheet from the oven and let the ingredients cool slightly. After cooling, remove the skins from the tomatoes for a smoother sauce.

In a large pot or Dutch oven, sauté the onions over medium heat until they're soft and translucent. Add the roasted tomatoes and garlic to the pot, along with any juices from the baking sheet.

Blend the roasted ingredients using a stand blender or immersion blender until smooth. If you prefer a chunkier sauce, blend until you reach your desired consistency.

Simmer the blended sauce over medium heat for at least 30 minutes to allow it to reduce and thicken. Stir occasionally to prevent sticking.

Taste the sauce and adjust the seasoning as needed by adding more salt, pepper, or optional ingredients like chicken broth, parmesan, or cream. If using fresh herbs, add them at this stage for added flavor.

Once the sauce has reached your desired thickness, remove it from the heat and let it cool. Store in jars and refrigerate or freeze for later use.