Heat the oil in a large skillet and cook the onions and red pepper flakes (on medium heat) until onions are translucent, about 3-5 minutes. Add the garlic and cook for an additional minute.
Add the curry powder and cook for an additional 2 minutes.
Mix in the fresh spinach a couple handfuls at a time. As the spinach cooks down, you will have enough room to add more. It only takes a few minutes to cook, once it is soft and wilted, it is good to go.
Mix in the sour cream/greek yogurt and the salt.