2 yellow onions
1 red pepper
1 green pepper
2 jalapeño peppers
2 garlic cloves
zest of 1 lime
1 tablespoon packed light brown sugar
2½ tablespoons Mexican spice mix (divided 1 tablespoon and 1½ tablespoons)
3 tablespoons extra-virgin olive oil (divided 1½ tablespoons and 1½ tablespoons)
1¼ pounds flank steak
½ teaspoon freshly ground black pepper
1 teaspoon fresh lime juice