Remove stem from tomatoes (once frozen, the stem is hard to remove). Freeze tomatoes overnight until rock hard.
Run tomatoes under warm water and remove the skin.
Place tomatoes in a medium-sized saucepan. Add water, sugar, salt, lemon juice, and garlic (if using). Cover and bring to a simmer. Cook for about 1 hour covered, then uncover and let steam escape, continue cooking until most of the liquid has evaporated, about 30 minutes.
Using an immersion blender, blend until smooth. I recommend transferring to a tall jar first since it blends more easily. Blend until smooth.
Your tomato sauce is ready to be used and can be refrigerated for about a week or frozen until ready to use. This recipe yields about 2 cups of sauce.