Preheat the oven to 425°F.
In a large bowl, whisk together the miso paste, oil, soy sauce, ginger, garlic, a pinch of black pepper, and red pepper flakes until well-combined.
Add the Broccolini to the bowl and toss with the miso mixture until it's well-coated.
Transfer the Broccolini to a baking sheet and arrange into a single layer.
Roast for 10 minutes, stirring once about halfway through.
Remove from the oven and serve immediately.
