For The Salsa: Saute the onion, jalapeño and garlic in a little olive oil just until soft. Transfer to the blender. Add all of the remaining ingredients to the blender. Blend on high until smooth. Preheat 2 tablespoons of olive oil to medium heat for 2 minutes. Add salsa to hot oil and cook on low for 20 to 25 minutes.
For Curtido: In a large skillet, heat 2 tablespoons of olive oil to medium heat. Add the onion, crushed red pepper flakes and salt to taste. Saute for a minute. Add the oregano, cabbage, carrots and vinegar. Stir well to combine. Season with salt, to taste. Remove from heat and let cool to room temperature.
For Masa: In a bowl, combine masa harina and salt. Gradually add in the water until dough forms. Knead the masa for a few minutes. If it's too dry add a little more water. And if it's too sticky, add a little more masa harina. Shape and form six equal balls. Keep covered with plastic.
Preheat a large griddle pan or skillet to medium heat for 5 minutes.
Take one ball and using your hands form a patty about 4 inches wide. You can use water or a little oil on your hands to make this easier. Place some filling in the center(about 2 full tablespoons) and form the dough around the filling to seal back up. Form into a patty once again, but try not to squeeze to hard or filling will come out. The patty should be about 4-5 inches wide.
Place the stuffed pupusas in the preheated surface. Cover with lid and cook for about 3-5 minutes per side or until you get some nice golden brown spots. Once you turn them, drizzle them with a little oil to get a crispy exterior. Adding the oil is optional. Remove form heat and let stand for a one minute before serving. Serve with your favorite salsa or with a pickled cabbage slaw, curtido.