This recipe for Swiss Tart with Caramelized Onion is a decadent and savory treat for a holiday or Sunday family dinner. This recipe is for four servings, so if you need to adjust it, make note. Ingredients include sugar, butter, egg yolks, cold water, salt, oil, sliced onions, buttermilk or milk soured with vinegar, eggs, grated mozzarella cheese, nutmeg, crumbled blue cheese, salt, and ground black pepper.