Heat the butter in a large non-stick skillet over medium heat. Saute the shallots until soft, about 3 minutes. Add the garlic and red pepper and cook for another minute. Sprinkle in the flour and stir to coat. Cook for a minute, then whisk in the salt and broth.

Simmer until thickened slightly, about 2 minutes. Add the spinach and carefully fold in until wilted. Stir in the Greek yogurt and taste for seasoning. Transfer to a serving dish and eat immediately.