COOK penne in a large pot of boiling water, following package directions but omitting salt, until tender, 10 to 11 min. Add chopped rapini (tough lower stems discarded) to water during the last 2 min of cooking pasta. Reserve 2 tbsp pasta cooking water in a bowl, then drain pasta and rapini.
HEAT olive oil in an extra-large frying pan over medium. Add garlic and cook, 1 min. Stir in pasta, rapini, reserved cooking water, sun-dried tomatoes, parmesan, lemon zest, lemon juice and salt until coated. Season with fresh pepper.
DIVIDE pasta among 4 plates. Sprinkle with remaining 2 tbsp parmesan.