In blender, purée parsley stems and oil until smooth. Strain through cheesecloth; set aside. (Can be refrigerated for up to 1 week).
Pasta:
Cook pasta according to package directions. Drain and reserve 1/2 cup (125 mL) cooking liquid.
Meanwhile, in large skillet set over medium heat, heat 2 tbsp (30 mL) parsley oil; cook lamb, garlic, salt and pepper for 5 to 8 minutes or until browned and cooked through. Stir in rapini and chili flakes; cook for 2 to 3 minutes or until heated through.
Toss together pasta, cooking liquid, half of the Parmesan cheese and lemon juice until well coated.
Divide among 6 pasta bowls. Garnish with remaining Parmesan and drizzle with remaining parsley oil.
Tips
For restaurant-style plating, twirl pasta together with tongs for each portion to make a neat plate of piled-high pasta.