Upside-Down Blueberry Muffin

Ingredients

• For the batter:
• 12 ounces all-purpose flour, such as Gold Medal
• 5 1/4 ounces plain or toasted sugar
• 2 teaspoons baking powder
• 2 teaspoons lemon zest, from 1 small lemon
• 1 teaspoon Diamond Crystal kosher salt; for table salt, use about half as much by volume or the same weight
• 1/4 teaspoon baking soda
• 1/4 teaspoon ground coriander seed
• 1/8 teaspoon grated nutmeg
• 6 ounces unsalted butter, cut into 1/2-inch cubes
• 5 ounces milk, any percentage will do
• 2 large eggs, straight from the fridge
• 2 teaspoons vanilla extract
• For the topping:
• 16 ounces fresh blueberries, rinsed and dried, see note for using frozen fruit
• 1 3/4 ounces sugar
• 1 1/2 ounces lemon juice, from 1 small lemon
• Salt, to taste

Directions

1. Place a rack in the center of the oven, then heat it to 350 degrees F (180 degrees C). Fit parchment into a 10-inch cast iron skillet and put it aside. Add flour, sugar, baking powder, lemon zest, salt, baking soda, coriander, nutmeg, and butter to a stand mixer’s bowl; attach the paddle. Mix on low until the mixture looks like a coarse, floury meal, about 2 minutes. If you want to work ahead, this mix can be sealed in an airtight container and left at cool room temperature overnight. When you’re ready to continue, scrape it into a large bowl, then stir in the milk, eggs, and vanilla (if using).
2. Pour the blueberries into the prepared skillet and gently shake the pan so the fruit settles into an even layer. Scatter the sugar, lemon juice, and salt over the berries. Spoon the batter across the top as evenly as you can, then level it with an offset spatula. Bake until the muffin has risen and the top is golden brown, about 45 minutes. The skillet’s weight and thickness can shift the timing, so watch it carefully toward the end to avoid baking it too long.
3. To serve, slide a dull knife around the inside edge of the skillet to release the muffin. Set a large plate or platter upside down over the skillet, then turn everything over together. Pause for about 30 seconds before lifting off the cast iron skillet, and be careful of the burst of steam. Slice into generous wedges and serve right away, since leftovers will become stale quickly overnight.