Place crushed saffron in a small bowl and add ¼ cup boiling water. Set aside.
Melt butter over medium heat. Add shallot, garlic, and ½ tsp salt. Cook until shallot is slightly softened, ~3-4 mins.
Add white wine and cook for ~3 mins, stirring occasionally. Add saffron liquid, red pepper flakes, and season with salt and freshly ground pepper. Cook, stirring often, until mixture is slightly reduced, ~5 mins. Add cream/half-n-half and yogurt and stir to combine.
If including - add chopped spinach and cook until fully wilted and combined.