Preheat the oven to 425° F. Trim the stems off the broccolini and slice the onion into 1/2 inch thick strips.
Place the broccolini and onions in a medium-sized mixing bowl with olive oil, minced garlic, red pepper flakes, garlic powder, salt and pepper. Toss to combine.
Place the broccolini and onions on a baking sheet lined with parchment paper (you might need two), making sure they all have a little space in between so they can crisp. Roast the broccolini for 20-25 minutes, until it begins to char. Top with a squeeze of fresh lemon juice and Asiago cheese if desired.
Notes
See notes in post above for stovetop instructions for sautéed broccolini.