Bring a large pot of water to a boil. Cut the stem end out of each tomato. Cut a small, shallow X into the blossom (curved) end of each tomato. Plunge the tomatoes, in batches, into boiling water for 1 to 2 minutes. Lift out of the water and set in ice-cold water. Pull the skins off the tomatoes; they should easily slip off.
Cut each tomato in half, remove and discard the seeds, then coarsely chop and place in a bowl. Add any liquid from the cutting board to the bowl.
Heat the oil in another pot set over medium heat. Add the onion, bell pepper, celery, garlic and chili flakes and cook until vegetables are tender, about 5 minutes. Add the chopped tomatoes, stock, tomato paste and sugar and bring to a slow simmer. Simmer until the tomatoes are tender and falling apart, about 20 minutes. Stir in the basil, salt and pepper.
Coarsely puree the mixture in the pot with an immersion blender, or in a blender or food processor. Season the sauce with salt and pepper.
Divide cooked pasta among 4 bowls. Top with the marinara sauce, to taste, sprinkle with parsley, and serve. If you have marinara sauce leftover from the meal, it could be frozen for another time.
Note: Gluten-free pasta is available in our dried, bagged pasta aisle. Corn-based pasta was used in this recipe.