Trim ends of zucchini halves, and scoop out seeds and pulp, leaving about 1/2 inch pulp on skins. In a large saucepan, steam zucchini halves in a vegetable steamer over boiling water for about 5 minutes, until tender but firm. Drain, and immediately plunge into a large bowl filled with enough ice water to cover. Allow to cool completely; drain.
Melt 1 tablespoon butter in a medium saucepan over medium heat. Slowly cook and stir onion and garlic until onion is soft. Stir in spinach, and cook about 1 minute, until wilted.
Transfer spinach mixture to a medium bowl, and mix in ricotta cheese, eggs, Parmesan cheese, parsley, salt, basil, oregano and pepper. Stuff zucchini halves with even amounts of the mixture, and arrange in a large baking dish. Drizzle with remaining butter.
Bake uncovered in the preheated oven for 20 minutes, until the stuffing is hot and bubbly. Heat tomato sauce in a small saucepan, and spoon over the zucchini half way through baking.