Heat oven to 425°F. Cook pasta per package directions. Reserve 1 cup pasta cooking water; drain pasta and return to pot. On rimmed baking sheet, toss asparagus and garlic with 1 tbsp. oil and 1/4 tsp. each salt and pepper. Zest lemon and set zest aside. Halve lemon and place on baking sheet, cut sides down; roast 4 min. Meanwhile, toss shrimp with remaining Tbsp oil, then red pepper, 1/2 tsp. salt, and 1/4 tsp. pepper. Nestle shrimp on tray with asparagus and continue roasting until shrimp are opaque throughout and asparagus is just tender, 5 to 7 min. more. Transfer lemons to plate, then transfer vegetables, shrimp, and any pan juices to pot with pasta. Squeeze roasted lemon halves over top and toss to combine, adding some reserved pasta water if pasta seems dry. Toss with parsley and serve sprinkled with reserved lemon zest, grated Parmesan, and additional red pepper if desired.