Heat olive oil in a large skillet over medium heat. Add onion and garlic powder; cook and stir until onion is translucent, about 5 minutes.
Add tomatoes, sugar, basil, parsley, and salt; bring to a boil.
Reduce heat and simmer, stirring occasionally, until sauce thickens. 1-2 hours.
Notes
This recipe is courtesy of Katie Schroeder. Note from Katie: With a ton of fresh tomatoes at once, I typically double this and combine with 1 pound of spaghetti (boiled, drained, prepared).