In a large skillet, cover and cook spinach over medium heat for about 5 minutes. Drain and squeeze spinach well. Chop spinach; set aside.
In a large bowl, whisk together egg, ricotta and spinach. Gradually stir in 2½ cups (625 mL) flour and salt with wooden spoon until a soft dough forms. Place dough on a floured surface and knead in remaining flour for 5 minutes to form a smooth dough.
Divide dough into 8 pieces; roll each into long, thin strands, about 1 inch (2.5 cm) thick. Cut into about 36 1-inch pieces. Roll each piece across tines of a fork, gnocchi paddle or gnocchi basket. Place in a single layer on a floured baking sheet. Repeat with remaining dough. Place in the freezer for about 1 hour or until firm. Use a flat spatula or wooden spoon to gently scrape the frozen gnocchi off the baking sheet.
In a large pot of boiling salted water, cook gnocchi from frozen for about 8 minutes or until they float to the top and are tender throughout. Drain with a slotted spoon and toss with tomato sauce or butter.
Make ahead: If you want to freeze the gnocchi, once they are firmly frozen, use a spatula to scrape pasta off baking sheet into a resealable plastic bag and freeze for up to 3 months. To make plain ricotta gnocchi, omit spinach and use only 2 cups (500 mL) of flour to stir in to form soft dough. You may need to knead in more flour to form a smooth dough.
Makes 2 lb, enough for 4-6 servings.