Add water into a large pot and bring it to a boil. Add ½ tsp of salt.
When it starts boiling, add spaghetti.
Cook spaghetti for 8 minutes.
Drain almost all the water, but leave just a little bit on the bottom (about ½ cup).
Put the pot back on the stove at low temperature.
Add tomato juice and stir.
Add garlic cloves, salt, pepper, onion powder, and chili flakes.
Cook for 3 more minutes, while stirring occasionally.
Add basil and 1-2 handfuls of spinach. Stir and cover the fresh leaves with the hot pasta. It will steam in a couple of seconds.
Add the rest of the spinach and stir. Let it steam in the hot pasta.
Cook for 2 more minutes and remove from heat.
Add grated parmesan (optional), stir and cover with a lid.
You can serve it in about a minute.