Place the freekeh in a medium bowl, cover with cold water, and set aside.
Heat the olive oil in a large (4- or 5-quart) saucepan over medium heat.
Add the onion and cook, stirring, until softened, 8 minutes.
Add the kohlrabi and carrots and cook, stirring, until the vegetables begin to soften, 5 minutes; season generously with salt and black pepper.
Add the garlic and cook 1 more minute.
Drain the freekeh, rinse it with cold water, and add it to the pot.
Add the broth, zucchini, Parmesan rind if using, za’atar, salt, and the cayenne. Bring to a boil, then reduce the heat and simmer, uncovered, until the soup is thickened, 25 to 30 minutes (or a few minutes longer if you’re using whole freekeh instead of cracked freekeh).
Remove the Parmesan rind, season with more salt and black pepper to taste, divide among bowls, garnish with herbs, and drizzle with olive oil.