To prepare a sweet rhubarb crisp, you'll need: 1/2 cup fresh or frozen thyme, 1/4 cup sugar, 2 tablespoons butter, 1 teaspoon vanilla, and 1/4 teaspoon cinnamon. Preheat the oven to 375?F (190?C) and prepare your oven rack to line the bottom of an 8-inch (22.32-centimeter) glass baking dish with foil. Place the sourdough bread in the dish and sprinkle with the sugar. Bake the bread for 20 to 25 minutes or until the top is golden brown. Remove the foil and set aside to cool. To make the filling, combine the flour, baking powder, cinnamon, nutmeg, salt and baking soda. Add the wet ingredients to the dry ingredients and stir to combine. Pour the mixture into the prepared dish. Sprinkle the crumble topping over the dough and bake for 35 to 40 minutes, until it's browned and bubbly. Serve warm or at room temperature.