1Heat the oven to 375°F and arrange a rack in the middle. Coat a 2-quart baking dish with butter and set aside.
2Combine the flour, half of the half-and-half, the eggs, and the egg yolk in a large bowl and whisk until smooth. Add the cream and remaining half-and-half, season well with salt and pepper, and whisk to combine; set aside.
3Combine the potatoes, herbs, and zest in a large bowl and toss to coat; set aside. Heat the measured butter in a large frying pan over medium heat. When it foams, add the leeks and garlic and season with salt and pepper. Cook until the leeks are soft, about 5 to 6 minutes.
4Mix the cooked leeks into the potato mixture and season with additional salt and pepper. Evenly spread the potato mixture in the prepared baking dish and pour the egg mixture over top.
5Bake until the custard is puffy, golden, and set, about 30 to 35 minutes. Let rest at least 20 minutes before serving. The egg pudding can be served warm, at room temperature, or cold.
Beverage pairing:Chappellet Napa Valley Chenin Blanc, California. Tarragon and Chenin Blanc always seem to work together, since both have gentle herbal flavors and a certain inherent springy sweetness. The wine will also harmonize with the leeks and lemon zest. Chenin is one of the great white grapes of the Loire, but Chappellet has been making this beautiful version in Napa for years.