Warm Eggplant Salad with Walnuts

walnuts

fresh lemon juice (from about 1 lemon)

ground cinnamon

crushed red pepper flakes

plus 1 Tbsp. extra-virgin olive oil

medium Japanese eggplants (about 1½ lb. total), halved lengthwise, cut crosswise into 2" sections

small red onion, very thinly sliced

mint leaves, torn if large

Kosher salt

date syrup