Fill a large pot with about 2 inches of water and bring it to a boil over high heat.
Meanwhile, trim the broccoli crowns and cut into 2- to 3-inch florets.
When the water is boiling, set a steamer basket in the pot, add the broccoli, cover, and steam until just cooked and tender-crisp, 7 to 9 minutes.
While the broccoli steams, heat the olive oil in a small saucepan over medium-low heat. Add the pitted Kalamata olives, the finely chopped garlic, the kosher salt, and the dried oregano. Cook, stirring occasionally, until the garlic is lightly colored, 3 to 5 minutes. Remove the pan from the heat.
Finely grate the lemon zest and set the zest aside. Juice the lemon and add the lemon juice to the oil-and-olive mixture. Season the dressing with freshly ground black pepper.
Spread the steamed broccoli on a warmed serving platter. Reheat the dressing over medium-low just until it begins to bubble, then pour it evenly over the broccoli.
Sprinkle the broccoli with the reserved lemon zest, add additional freshly ground black pepper if desired, and serve immediately.
Notes
If you are preparing this dish asgluten-free, just be sure to use a brand of olives that is known to be GF.