Preheat oven to 425 °. Line a baking sheet with parchment paper.
Prepare the veggies by chopping the broccoli into florets and slicing the onion into half-moon shaped pieces. Toss the broccoli & onion in the olive oil, garlic, and red pepper flakes. Transfer to the baking sheet.
Bake for 20 to 25 minutes or until the broccoli is slightly charred. Garnish with lemon zest & juice. Serve.