Rinse the rice very well under cold water, drain and set aside.
Heat the oil in a dutch oven or large pot over medium-high heat. Add the onion and cook until softened, about 3 minutes.
Add the spinach and cook until wilted, about 2 minutes.
Add the rice, vinegar, cumin, 1/2 tsp salt, and 1/4 tsp pepper and cook while stirring about 1 minute.
Stir in the water and bring to a boil. When water begins to boil cover the pot and reduce heat to low. Cook, WITHOUT REMOVING THE LID AT ANYTIME, for 20 minutes. Take off the heat, KEEP THE LID ON, and let sit for 15 more minutes to allow the rice to absorb all the liquid.
Remove the lid, add the lemon juice and gently toss with a fork. Season with salt and pepper to taste.