Heat olive oil in a medium saucepan over medium-high heat. Add onion; stir to coat with oil and cook until fragrant and turning translucent, about 5 minutes. Add garlic, Italian seasoning, salt, pepper, and red pepper flakes. Cook and stir for 1 to 2 minutes.
Stir meatless crumbles and balsamic vinegar into the saucepan. Cover and cook until crumbles are mostly thawed, about 5 minutes. Add crushed tomatoes and bring to a boil, stirring frequently. Taste for seasoning and adjust accordingly. Add agave nectar.
Pour in pasta sauce, 1 jar at a time, stirring well. Cover and bring to a boil. Reduce heat to medium-low and simmer until flavors combine, at least 1 hour.
