Return pan to medium heat and add remaining 3 tablespoons of butter. Add onion and shallots and cook until translucent, about 5 minutes. Then add garlic and a pinch of chili flakes. After a minute or two, add the lemon zest and stir for a minute. Add the salt, lemon juice, and wine and bring to a simmer. Allow the liquid to reduce down to about half a cup. Reduce heat to medium-low.