When the oil is hot, add the onion and cook, stirring frequently, until softened, about 5 minutes.
Add the garlic and cook, stirring, for 1 minute.
Add the Italian seasoning and cook, stirring, for 1 minute more.
Add the tomatoes, salt, and pepper, stir to combine, and bring to a simmer. Reduce the heat and simmer for 10 minutes or up to an hour.
Serve topped with crushed red pepper flakes if desired in bowls for dipping your favorite appetizers and snacks such as mozzarella sticks, fried ravioli, or garlic bread.
Notes
Tips for customizing marinara dipping sauce
Swap the crushed tomatoes for canned diced tomatoes
Instead of Italian seasoning, try a mix of your favorite fresh Italian herbs such as oregano and rosemary (use 3 teaspoons of fresh herbs instead of 1 teaspoon dried)