Heat the oven to 375°F and arrange a rack in the middle. Coat a 2-quart baking dish with butter and set aside.
Combine flour, 1/2 of the half-and-half, eggs, and egg yolk in a large bowl and whisk until smooth. Add cream and remaining half-and-half, season well with salt and freshly ground black pepper, and whisk to combine; set aside.
Combine potatoes, herbs, and zest in a large bowl and toss to coat; set aside. Heat butter in a large frying pan over medium heat. When it foams, add leeks and garlic, and season with salt and freshly ground black pepper. Cook until leeks are soft, about 5 to 6 minutes.
Mix cooked leeks into potato mixture and season with additional salt and freshly ground black pepper. Evenly spread potato mixture in the prepared baking dish and pour egg mixture over potatoes.
Bake until custard is puffy, golden, and set, about 30 to 35 minutes. Let rest at least 20 minutes before serving. Can be served warm, room temperature, or cold.
Beverage pairing: Chappellet Napa Valley Chenin Blanc, California. Tarragon and Chenin Blanc always seem to work together, since both seem to 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.
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