Preheat oven to 300°F. Line a baking sheet with parchment paper or Silpat mat.

In a microwave-safe bowl combine onion, garlic, and olive oil. Microwave for 30 seconds to infuse the garlic and onion into the oil. When you remove the bowl from the oven you will smell the heavenly aroma of garlic and onion.

Add salt, pepper, Italian seasoning and sugar to bowl and stir to combine. Sugar will help cut the acidity of the tomatoes.

Place cut tomatoes in a large mixing bowl, pour in the olive oil mixture over top and toss with your hands or a spoon. Make sure all of the tomatoes are well coated.

Place the tomatoes cut side up on the baking sheet. Sprinkle with the fresh basil, rosemary, and any remaining olive oil mixture from the prep bowl.

Place tray on middle rack in oven. Cook for 2 hours. After 2 hours of slow roasting, your tomatoes will look slightly caramelized.

Turn the oven up to 400°F and roast for 30 minutes, or until the tomatoes start to brown. They will be caramelized and delicious. If you prefer a brighter tomato sauce pull them out before they brown. Approximately 10-15 minutes after you turn the oven to 400°. These roasted for the full 30 minutes, you can see the edges of the tomato are nice and browned.

Remove pan from oven and transfer tomatoes to the bowl of a food processor. Process until tomato sauce is at your desired consistency. I like mine saucy, not too chunky.