Preheat oven to 375 degrees. Butter bottom and sides of four 4-inch, loose-bottomed tartlet pans. Cut out four 6-inch circles from the sheets of puff pastry and press them into prepared pans. Prick pastry bases with a fork. Line pastry with baking paper, fill with baking beads or dried beans and bake for 15 minutes. Remove from oven, remove weights and baking paper and return tart pans to oven for 5 additional minutes. Remove from oven and transfer pans to a wire rack. In a frying pan over medium heat, warm olive oil. Add onions and cook for 5 minutes. Stir in arugula and remove from heat. In a small bowl, combine eggs, ricotta and nutmeg. Season with salt and pepper to taste. Beat lightly, leaving some of ricotta in lumps. Stir in onion-arugula mixture. Spoon mixture into pastry shells and bake until set, about 25 minutes. Remove from oven to a wire rack, cool 5 minutes, then lift pastry from pans and serve.