How to Make It
Heat oil in a 13-inch paella pan or large skillet over medium-high. Add onion and garlic; cook, stirring constantly, until tender and lightly browned, about 30 seconds. Add carrots, bell pepper, cauliflower, zucchini, and mushrooms; cook, stirring occasionally, until softened, about 5 minutes.
Crumble saffron over vegetables in pan (if using). Stir in stock and salt. Increase heat to high. Bring to a boil; cook 3 minutes. Stir in rice and green beans. Reduce heat to medium-high; cook 6 minutes. Reduce heat to medium; cook until rice is tender but still has a bite, about 12 minutes. (During the last few minutes of cooking, you should hear rice crackling at the bottom of the pan.) Remove paella from heat; cover with a clean kitchen towel, and let stand 5 minutes. Garnish with thyme or parsley leaves, if desired.