Preheat oven to 300ºF
Line a baking sheet with parchment paper.
In a microwave safe bowl combine onion, garlic & 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 & onion.
Add salt, pepper, italian seasoning and sugar to bowl and stir to combine. Sugar will help to cut the acidity of the tomatoes.
Place cut tomatoes in a large mixing bowl, pour in the olive oil mixture over and toss with your hands or a spoon. Make sure all of the tomatoes are well coated.
Place 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 400ºF 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. Approx 10 - 15 minutes after you turn the oven to 400º
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.