This dish is best made with a creamy variety of feta, which melts perfectly into the warm pasta. Use a sheep-and-goat's-milk blend if you can find it; otherwise, a good fresh goat cheese or French feta works well.
Preheat oven to 375 degrees. Place grape tomatoes in a 9-by-13-inch baking dish, drizzle with 1 1/2 tablespoons olive oil, and season with salt and pepper. Roast until tomatoes have collapsed and are beginning to brown, about 45 minutes.
In a heavy skillet over high heat, add remaining olive oil and the onions. Cook until beginning to brown, about 5 minutes. Reduce heat to medium-low, and cook for 25 minutes. Add a pinch of salt and cook until caramelized, about 5 minutes.
Cook pasta until al dente. Drain pasta, and return to pot; add tomatoes and caramelized onions. Crumble 1 cup of feta over pasta, and gently toss.
Divide among 4 plates, and garnish each with remaining feta and a drizzle of olive oil.