In a blender or food processor, combine the flour, soy milk, stock, olive oil, garlic, thyme, mustard and pepper. Process until smooth. Pour the batter into a large bowl. Let stand for 1 hour in the refrigerator.
Heat the oven to 425 F. Lightly coat a 9-by-13-inch baking pan with cooking spray.
In a large, spotlessly clean bowl, using an electric mixer on high, beat the egg whites until stiff peaks form. Gently fold the egg whites into the batter.
Pour the batter into the prepared pan. Bake until puffed and lightly browned around the edges, about 15 minutes. Let cool for 15 minutes.
Heat the broiler. Position the rack 4 inches from the heat source.
While the polenta is cooling, make the topping. In a small frying pan, heat the olive oil over medium-high heat. Add the onion and cook until soft and lightly golden, about 6 minutes. Add the olives and tomatoes and cook for 1 minute. Remove from the heat.
Carefully spoon the onion mixture evenly over the baked polenta and sprinkle with the cheese. Broil until the top is lightly browned. Watch carefully; this takes only about 1 minute. Sprinkle with the parsley. Transfer to a wire rack and let cool for 10 minutes. Cut into 8 squares, then cut the squares on the diagonal into 16 wedges. Serve immediately.