Place the 2 Tbsp. olive oil in a pot set over medium heat. Add the onion, bell pepper, celery and garlic and cook 4 to 5 minutes. Add the tomatoes, stock or water, sugar, chili flakes, thyme and bay leaf. Bring to a simmer, loosely cover, and simmer the sauce 15 minutes; season to taste with salt and pepper.
Preheat the oven to 425 degrees F. Place the snapper in a single layer in a parchment paper-lined baking dish. Drizzle with the 1 Tbsp. olive oil and the lemon juice; season with salt and pepper. Bake 12 to15 minutes, or until cooked through. Arrange 2 pieces of fish on each of 4 plates, top with sauce, sprinkle with parsley and serve.