Preheat oven to 425 degrees F. Line a baking sheet with parchment paper.
Prepare the veggies by chopping the heads of broccoli into florets and slicing the red onion into half-moon shaped pieces. Toss the broccoli and 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 florets are slightly charred. Sprinkle with lemon zest and a squeeze of fresh lemon juice before serving. Enjoy!