Cook pasta according to package directions. Drain. Cover and keep warm. Meanwhile, in a large skillet cook onion and garlic in hot olive oil over medium heat until onion is tender. Add tomatoes, salt, black pepper, and, if desired, red pepper. Cook and stir over medium-high heat about 2 minutes or until the tomatoes are warm and release some of their juices. Stir in arugula, watercress, or spinach and heat just until greens are wilted.
To serve, divide pasta among individual serving bowls. Top with tomato mixture. Sprinkle with toasted pine nuts or almonds and cheese. Makes 4 main-dish servings.