Heat the oven to 375°F and arrange the rack in the middle. Butter a 2-quart baking dish and set aside.
Whisk flour, 1/2 of the half-and-half, eggs, and egg yolk in a large bowl until smooth. Add remaining half-and-half and cream, 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. Once foaming, add leeks and garlic, and season with salt and pepper. Cook until leeks are soft, about 5 to 6 minutes.
Add cooked leeks to potato mixture, season with additional salt and pepper, and stir to combine. Evenly arrange potato mixture in the prepared baking dish. Carefully 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.
