Preheat the grill to high heat (425℉). Soak 5-6 wooden or bamboo skewers in water for 30 minutes if using.
Steam the Brussels sprouts for 4-5 minutes and let them cool for 3-4 minutes.
Thread the steamed Brussels sprouts onto the skewers, inserting the pointed end into the stem.
In a small bowl, whisk together the maple syrup, mustard, oil, garlic, red pepper flakes, black pepper, and salt. Set aside 1 tablespoon of the glaze.
Brush the glaze onto the skewered sprouts; use a spoon if necessary to slather it on.
Grill the skewers directly on the grate for 2 minutes until charred. Turn and repeat for a total of 6 minutes.
Remove from the grill, drizzle with reserved glaze, and serve hot or cold.
Store leftovers in an airtight container in the fridge for up to 4 days.