Preheat the oven to 350 °F and set a rack in the middle of the oven. Grease a 9-inch baking pan or cast-iron pan with butter.
Place 1 pint fresh blueberries in a medium-sized bowl, along with 1 tablespoon of sugar and the lemon zest. Toss to combine and set aside.
Use a small paring knife to slice ½ vanilla bean vanilla bean in half lengthwise. Turn the edge of the knife perpendicular to the vanilla bean and scrape the seeds out.
In a large blender cup, add the vanilla bean seeds and the remaining ⅓ cupgranulated white sugar. Crack open and add 3 large eggs, along with 1 tablespoon unsalted butter, ⅛ teaspoon kosher salt, ¼ teaspoon almond extract, and ⅔ cup all-purpose flour.Use a fork or whisk to whisk the mixture until homogeneous (it will be slightly lumpy - that's okay).
Whisk in ½ cup heavy cream and ¾ cup whole milk. Use a hand blender or regular blender to blend until fully homogeneous, frothy, and no lumps remain. Set aside.
Arrange the sugared blueberries evenly on the bottom of the pan. Slowly and evenly pour in the clafoutis batter.
Bake the clafoutis for 30 - 35 minutes until the edges are golden-brown and the middle is just set, but still slightly wobbly. I recommend checking every 5 minutes after 25 minutes. A toothpick placed into the center of the pan should be clean.
Remove the clafoutis from the oven and let cool for 10 - 15 minutes until still warm, but not hot. Dust with powdered sugar, cut into slices, and serve warm.
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Notes
Metal baking pans can react with acidic fruits, leaving a metallic taste. Cast-iron pans, ceramic bakeware, and glass bakeware should be totally fine!
Clafoutis can have a rubbery texture when overcooked. To avoid this, begin checking the clafoutis at 25 minutes; the edges should be golden-brown and the middle should be just set and slightly wobbly (but not wet or raw inside).
Leftover clafoutis can wrapped in plastic wrap (or an airtight container) and stored in the refrigerator for 3 to 4 days. To reheat, place a slice of clafoutis on a microwave-safe plate, and microwave for 30 seconds at a time until warmed through. It gets hot quickly, so keep checking!