Heat oil2 tablespoons in a large skillet over medium-high heat. Cook onion1, stirring often, until lightly charred and softened but not falling apart, 5–7 minutes. Add red pepper flakes1 teaspoon and toss to combine. Remove pan from heat and mix in lime juice1/4 cup and honey2 teaspoons. Let cool, then mix in lime zest1 teaspoon.