Combine tomatoes and their juices, 3 tablespoons oil, thyme, ¼ teaspoon red pepper flakes and pinch salt in a large serving bowl.

In a medium saucepan, combine onions and broth. Bring to a simmer and cook until onions are very soft, about 5 minutes. Puree in a blender with remaining tablespoon oil and pinch red pepper flakes until smooth.

Bring a large saucepan of salted water to boil. Add pasta and cook until al dente. About 2 minutes before pasta is done, stir in broccoli, peas and asparagus. Drain pasta and vegetables; add to bowl with tomato mixture. Add spring onion sauce to pasta and toss well. Season with salt and serve at once.