Combine the zucchini and salt in a colander and toss to mix. Set aside to drain for 30 minutes. Squeeze out the excess water. Preheat the oven to 350 degrees. Grease a 7- by-11-inch baking dish with butter. Stir together the flour and baking powder in a medium bowl. Add the onion, zucchini, cheese, thyme, and pepper. Mix well with a fork, breaking up any clumps of zucchini. Whisk together the oil and eggs in a small bowl. Pour into the zucchini mixture and mix well. Spread evenly in the baking dish. Bake for about 35 minutes, until golden. Let cool on a wire rack for 5 minutes. Cut into squares and serve warm or at room temperature. Serves 6.