Cook pasta until firm but tender. Drain and rinse under cold water; drain again very well.
Whisk together oil, vinegar, mustard, salt, pepper and garlic. Add pasta, red pepper, onions, feta cheese and half of the shredded basil; toss gently. Stack spinach leaves and roll up tightly; cut crosswise into thin strips.
Add to salad along with tomatoes and remaining basil; toss gently.