Preheat oven to 425 degrees. Lightly butter a 2-quart oval baking dish. Melt 2 tablespoons butter in a large pot over medium heat. Add leek, onion, 2 1/2 teaspoons salt, and 1/2 teaspoon pepper and cook, stirring, until translucent, about 10 minutes. (Do not let brown.)
Add rice and stir to thoroughly coat. Cook, stirring occasionally, until you hear the rice toasting (it sizzles and pops), about 2 minutes. Add wine and increase heat to medium-high. Simmer until pot is almost dry, about 3 minutes. Add 3 cups broth, bring to a simmer, and transfer to baking dish. Bake risotto 5 minutes and stir. Bake another 5 minutes and stir again, then bake until liquid is absorbed and rice is just tender, about 5 more minutes.
Remove risotto from oven and stir in remaining tablespoon butter, Parmesan, herbs, and lemon zest. Gradually add 1/2 cup warm broth, stirring, until rice is creamy. (For creamier rice, add more broth, 1 tablespoon at a time, until it reaches desired consistency.) Season to taste with salt and pepper and serve immediately. If risotto thickens upon sitting, add more broth to loosen as desired.