Preheat a Belgian waffle iron and grease or oil it according to the manufacturer's instructions.

In a medium bowl, whisk together 2 cups all-purpose flour, 1/2 tsp baking soda, and 1/2 tsp salt.

In a large bowl, whisk together 1 1/4 cups buttermilk, 1/2 cup milk, 2 Tbsp vegetable oil (or melted cooled butter), 2 Tbsp applesauce, the 2 egg yolks, 2 Tbsp granulated sugar, 1 1/2 Tbsp lemon zest, and 2 Tbsp lemon juice until combined.

In a small, clean bowl, use an electric hand mixer to beat the 2 egg whites until soft peaks form.

While whisking the wet mixture, slowly add the flour mixture and whisk just until combined — the batter should be slightly lumpy. Do not overmix.

Using a rubber spatula, gently fold the beaten egg whites into the batter until just combined.

Cook the waffles: spoon the appropriate amount of batter for your waffle iron onto the preheated iron (about 3/4 cup per waffle is a common guideline). Close the iron and cook according to the manufacturer's directions until the waffles are golden and crisp. Open the iron slowly when removing each waffle to avoid splitting. Repeat with remaining batter.

To make the blueberry syrup, place 1/3 cup granulated sugar and 2 Tbsp cornstarch in a medium saucepan and whisk to combine.

Stir in 1 cup cold water, then add 2 cups blueberries and 1 1/2 Tbsp lemon juice.

Cook the mixture over medium-high heat, stirring frequently, until it comes to a boil. Once boiling, reduce heat to medium-low and let it gently boil for 1 minute, stirring constantly.

Remove from heat and either (a) pour the hot mixture into a blender, cover, and blend on low about 1 minute until smooth, or (b) press the mixture through a fine-mesh sieve into a bowl if you prefer to remove skins and seeds. Allow the syrup to cool slightly.

Serve the waffles warm topped with the blueberry syrup. Store any leftover syrup in the refrigerator for up to one week.