In a large dutch oven or heavy bottom sauce pan heat olive oil on medium heat. Add in chopped onion and cook for 8-10 mins or until translucent. Add in minced garlic and herbs and cook for 1-2 mins or until fragrant. Add in San Marzano tomatoes, crushing them in your hand before adding them to the pan. Add in tomato paste, chili flakes, garlic salt, pepper, and bay leaf. Stir together and let simmer for 20-30 minutes, stirring frequently. You can take an immersion blender to the sauce after cooking or leave as is (the photos above are a chunkier consistency).
Preheat oven to 350 degrees.
In a large casserole dish add in layers as follows:
Top the very top layer with mozzarella cheese and freshly sliced tomatoes. Bake for 40-60 minutes or until the tomatos are roasted and the top is golden.