Preheat the oven to 375°F (190°C). Lightly grease an 8-inch (20 cm) round baking pan with olive oil or cooking spray.
Add the blueberries to the prepared pan and squeeze the lemon juice over the top. Toss to combine.
In a mixing bowl, use your hands to rub the lemon zest into the sugar until fragrant. Add both flours and whisk to combine, then create a well in the center.
Add the eggs to the well and stir until fully combined. The batter may look coarse and crumbly depending on your flour blend and egg size — that is normal. Keep stirring until the eggs are fully incorporated.
Spread the batter over the blueberries in the pan, covering as much surface as possible. Scatter the minced walnuts over the top, then pour the melted butter evenly over everything.
Place a sheet of aluminum foil or a drip pan on the oven rack below to catch any berry juice that bubbles over. Bake for 45-50 minutes, until the top is lightly browned and the filling is bubbling.
Cool for at least 10 minutes before serving. The filling is very hot straight from the oven and needs time to set up.
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Notes
Fruit swap: This cobbler works beautifully with any fruit, fresh or frozen — peaches, cherries, mixed berries, or whatever is in season.
Extra crust: Double the topping ingredients for a thicker, more substantial crust over the filling.
Frozen blueberries: California blueberries freeze exceptionally well. To freeze fresh berries, spread in a single layer on a baking sheet (do not wash first), freeze until solid, then transfer to an airtight freezer bag. Wash just before using.
Walnut storage tip: Store California walnuts in the refrigerator for up to 1 year or the freezer for up to 2 years. High oil content causes them to go rancid quickly at room temperature.
Serving suggestion: Serve warm with a scoop of vanilla ice cream or a spoonful of whipped cream.