gateway vegetable
n. A palatable vegetable that encourages more adventurous vegetable consumption.
We call pea shoots the "gateway vegetable," because when we grow them in the classroom, children just eat them by the fistful until green juice is running down their faces.
—Emily Armstrong, “Harvest of the Month: Pea shoots,” Martha's Vineyard Times, February 23, 2015
Healthy eating and weight loss requires vegetables. There's no way around it. Some of us would rather fill our plates with meat and bread. If you discover a "gateway vegetable" you'll start eating them more often.
—Dave Greenbaum, “Discover Your 'Gateway Vegetable' To Start Eating Healthier,” Lifehacker Australia, October 12, 2014
Messing with a stalwart like the spud doesn't go well with the potato industry, school cafeteria directors and legislators from potato-growing regions. They're fighting to see that in schools, no potato is left behind.

As part of the effort, spud sellers are promoting potatoes as a "true gateway vegetable" that could lead kids to broccoli.
—Jennifer Levitz & Betsy McKay, “Spuds, on the Verge of Being Expelled, Start a Food Fight in the Cafeteria,” The Wall Street Journal, May 17, 2011
2005 (earliest)
Onions are a gateway vegetable, they always lead to more dangerous vegetables.
—RobFromGa, “Monkeys on hunger strike, want onions (Addiction)” (comment), Free Republic, March 19, 2005
To reduce the percentage of frequent users of three gateway drugs (tobacco, alcohol, and marihuana) by 15 percent among 8- to 20-year-olds.