Rinse the tomatoes under cool running water and cut into rough chunks, removing stems and blemishes.

Combine chopped tomatoes, onion, garlic, basil, parsley, and 2 cups water in a large pot over medium-high heat.

Bring the mixture to a rolling boil, then reduce heat to medium and simmer for 10 to 12 minutes until tomatoes are soft and falling apart.

Remove the pot from heat and let the mixture cool for 5 minutes before blending.

Blend the tomato mixture until smooth using an immersion blender for 1 to 2 minutes or blend in batches in a regular blender.

Pour the blended mixture through a fine-mesh strainer lined with cheesecloth into a bowl, pressing gently to extract all the liquid.

Stir in sea salt, black pepper, red pepper flakes, and vinegar or lemon juice. Taste and adjust seasoning as needed.

Pour the juice into clean glass jars, refrigerate for at least 1 hour, and serve chilled.