In a big pot, cook down the onion and garlic in the olive oil over medium heat. Let the onions get translucent and fragrant.
Add the fresh and canned tomatoes and stir. Stir occasionally over the next 10 minutes.
Use an immersion hand blender to get this preliminary mixture smooth.
Add in the fresh herbs, seasonings, balsamic vinegar and sugar. Stir and cover for another 20 minutes, checking on it occasionally.
I added the cornstarch once I knew the marinara was finished because it seemed too water-y for me. Dissolving it in water first helps to eliminate lumps!
Let cool completely before storing.