"Chopped onion caramelized in butter gives these muffins a sweet and savory flavor. The prosciutto adds a hit of salt and richness. Serrano ham or other cured ham can be used in place of the prosciutto, if preferred. Make Ahead Tip: Prepare Steps 2 & 3, keeping the onion mixture, dry and wet ingredients separate; cover and refrigerate for up 1 day. Continue with Step 4 just before baking...."