Seems like training with a powermeter and using headwinds are the two suggestions for climbers living in Flatlands.
„… set about increasing your average wattage. Mr. Vande Velde does this by punctuating rides with five minute “power bursts,” dropping into a big gear, pushing his pedals as hard as he can, his wheels turning at his top sustainable watts and barely 50 revolutions per minute. In the next five minutes, he’ll click up into an easier gear, pedals whirring at low watts and about 90 r.p.m. (which any recreational rider should be able to maintain). Then he’ll repeat.“
„… if you live in a flat area, listen to local weather reports and note the direction of the wind. A strong head wind can simulate hill climbing, Mr. Vaughters said. “You need to push hard into the wind,” using “a big gear, for at least six minutes and no longer than 45 minutes” once or twice a week “to get the aerobic benefits of climbing big hills,” he said. (In recompense, you get a tail wind all the way home.)“