olive oil • sprigs of fresh rosemary • medium yellow onion, chopped (about 1 cup) • garlic cloves, minced (about 1 tablespoon) • low-sodium vegetable stock, plus more as needed for reheating • medium head cauliflower, cored and broken into 1½-inch florets (about 2½ pounds) • Optional: 1 can of chickpeas, drained OR 2 potatoes, peeled and diced • kosher salt (such as Diamond Crystal), plus more to taste • black pepper, plus more to taste • Freshly grated zest of 1 lemon, for serving • diced rustic country bread (¾-inch pieces)