Add trimmed and halved Brussels sprouts to a large mixing bowl (reserve garlic cloves for later use) and season with oil, salt, and pepper. Toss to coat and set aside.
In a separate small mixing bowl, add the miso glaze ingredients and stir / whisk to combine. Taste and adjust flavor as needed, adding more salt or miso for saltiness, vinegar for acidity, or maple syrup for sweetness. It should be equal parts tangy, salty, and sweet. Set aside for use after the Brussels sprouts are roasted.
Heat a large (at least 12-inch) oven-safe skillet (we prefer cast iron) over medium-high heat. Once hot, add cooking oil (it should coat the bottom of the pan, so add more as needed).
Wait 1 minute for the oil to heat, then add Brussels sprouts (NOT THE GLAZE — we’ll use that later). Make sure the pan is not crowded — all sprouts should have room to lie face down and they shouldn’t be piled on top of each other. If it’s crowded, remove a few to cook in a separate batch.
Cook for 2-3 minutes cut side down to get a sear. Once golden brown, add garlic, toss, and transfer to oven.
Bake for 10-15 minutes, removing from the oven every 5 minutes to toss and inspect the sprouts. You want them crispy and deep golden brown but not burnt.
Remove from oven and immediately add HALF the miso glaze (alternatively, reserve all as a dipping sauce). Toss to coat and serve hot with remaining glaze on the side for dipping.
Best when fresh. Store cooled leftovers covered in the refrigerator up to 2-3 days. Not freezer friendly. Reheat in the microwave or on the stovetop in a skillet over medium heat until hot.