Five years ago, when Valerie Onor, of Valerie Onor Interior Design, was looking for a new home in New York City, she told her husband she would love a little outdoor garden space, never imagining she would end up with 1,500 square feet of lush green and richly flowered rooftop splendor in the middle of Manhattan’s upper east side. “It’s a magical place,” Onor admits, “with lively color against a backdrop of varied...