Begin by heating the olive oil in a sauté pan over medium heat. Once the oil is shimmering, add the diced onion. Cook for about 5 minutes, or until the onion is translucent and fragrant. Then, add the minced garlic and sauté for an additional minute, being careful not to let it burn.
Transfer the sautéed onion and garlic mixture to your slow cooker. This will be the flavor base of your marinara sauce.
Open the cans of Hunt's Diced Tomatoes and Hunt's Tomato Paste. Pour both into the slow cooker, stirring well to combine with the onion and garlic mixture.
Add the bay leaves, dried basil, dried oregano, brown sugar, salt, black pepper, and crushed red pepper (if using). Stir thoroughly to incorporate all the seasonings into the tomato mixture.
Cover the slow cooker and set it on low. Allow the sauce to cook for 6 to 8 hours. This slow cooking process allows the flavors to deepen and develop beautifully.
Once the cooking time is complete, remove the bay leaves. Taste the sauce and adjust the seasoning with additional salt, pepper, or sugar as necessary. Serve the marinara sauce over your favorite pasta, use it in a lasagna, or freeze for later use.