Heat oven to 300F.
Line a baking sheet with a silpat r parchment paper. Arrange the tomato halves on the sheet, cut-side up. Sprinkle with sugar, oregano, salt, and pepper. Roast for 90 minutes, or until shriveled and dry.
Prepare the pasta according to the directions on the package. Reserve 1/4 cup pasta water before draining. While the pasta is cooking, melt the butter in a small saucepan over medium heat. Add the leeks and water and cook, stirring occasionally, until the leeks are very soft and almost all of the water has evaporated. Transfer leeks and any remaining liquid to a small bowl. Add the chopped fennel to the saucepan and cook for 5 minutes, or until it begins to soften. Reduce the heat to low. Add the olive oil and fennel seeds, and cook another 5 minutes.
Add the drained pasta, reserved pasta water, and leeks back into the pasta pot. Stir in the fennel, oil, dried tomatoes, and feta. Season to taste with salt and pepper.