Heat oven to 425°F. Divide and trim the broccoli florets and stalk into bite-size pieces. In a large bowl, toss the broccoli with the sliced garlic, 1 1/4 tsp. salt, 1/2 tsp. black pepper, and 1/4 tsp. pepper flakes. Drizzle 2 tablespoons of oil over the broccoli and mix with your hands, rubbing a thin coat of oil onto the broccoli and garlic.
Spread the broccoli in a rimmed baking sheet in an even single layer and roast for 15-20 minutes, until crisp-tender and the edges of the broccoli and garlic are browned.
In a mortar and pestle, crush pistachios until some are the size of bread crumbs mixed with larger, coarse pieces (or put the pistachios in a bag and use a meat mallet).
Transfer the broccoli and garlic to a serving bowl and toss the Parmesan, pistachios, lemon zest, and lemon juice. Serve immediately.