In a food processor, combine all the dough ingredients. Process for a few quick minutes until dough forms. The dough should not be sticky and should be easily removable from machine. (If it is sticky, add more flour by the tablespoon).
Turn dough out onto lightly floured work surface and form an oblong log. Wrap securely in plastic wrap. Let dough rest on work surface at least one hour (up to four hours).
In a small frying pan, saute the onions in olive oil for about six minutes on medium-high heat.
Once the onions have started to soften, add spinach and mushrooms. Cook for about five minutes longer, mixing everything together thoroughly.
Pour the vegetable mixture into a bowl. Add the garlic powder and combine thoroughly. Set aside.
After an hour, unwrap the dough from the plastic wrap and cut into four even portions. Flour work surface and rolling pin. Roll out one portion of dough into a thin nine inch square. Flour both sides as needed when rolling.
Cut out as many squares as you can from the rolled out dough. Brush a light coating of vegetable broth around the edges.
Place one rounded teaspoon of filling in center of one square. Top with a second square of dough and press the edges together to seal tightly. Dust ravioli lightly with flour. Repeat procedure with remaining dough.
Bring a large saucepan filled with the vegetable broth and water to a rolling boil. Add the raviolis, five to six at a time.
Cook each batch for three to four minutes. Serve hot with your favorite melted butter sauce.