3 tablespoons vegetable oil
2 teaspoons kosher salt
Peel jicama with a pairing knife then cut into 1/4-inch-thick slices (this is most easily done on a mandoline). Stack a few slices of jicama on top of one another and cut into 1/4-inch-thick strips. Repeat with remaining jicama slices.
Slice cabbage 1/4-inch thick (also easily done with a mandoline). Toss cabbage and jicama together in a large bowl. Cover and refrigerate until ready to serve.
To make the dressing, combine lime juice, jalapeños, cilantro, scallions, vegetable oil, and salt in a medium bowl.
When ready to serve, toss the vegetables and dressing together until well-coated. Taste and add more salt or lime juice, if desired.