Trim the Brussels sprouts, halve them, and toss with the olive oil, 1 tablespoon of the date syrup, the walnuts, and the salt. Lightly oil the hot sheet pan, then spread the Brussels sprouts in an even layer.

Roast on the top rack until crisp and browned, 30 to 35 minutes, without flipping.

Stir the remaining ½ tablespoon date syrup with the cinnamon. Transfer the Brussels sprouts to a serving plate, drizzle with the syrup mixture, and finish with flaky salt.