Step 1

Arrange a rack in the middle of the oven and heat to 375°F. Line a standard 12-well muffin pan with paper liners or coat the wells with cooking spray.

Step 2

Whisk the oat flour, almond flour, sugar, poppy seeds, baking powder, baking soda, and salt together in a large bowl, breaking up any large clumps of almond flour; set aside.

Step 3

Whisk the yogurt, oil, egg yolks, lemon zest, lemon juice, and vanilla together in a medium bowl until combined; set aside.

Step 4

Place the egg whites in the bowl of a stand mixer fitted with the whisk attachment. (Alternatively, use an electric hand mixer, small food processor, or sturdy whisk). Beat on medium-high speed until stiff peaks form, 2 to 3 minutes.

Step 5

Pour the yogurt mixture into the almond flour mixture and stir with a rubber spatula until just combined. Some lumps are fine. Stir 1/3 of the beaten egg whites into the batter with the spatula. Then gently fold the remaining egg whites in until just combined (do not overmix).

Step 6

Divide the batter among the prepared muffin wells, filling each to the top (about 1/3 cup each). Bake until the muffins are golden brown and a toothpick inserted into the center of a muffin comes out clean, about 20 minutes. Cool the muffins in the pan for 10 minutes. Remove from the pan to a wire rack and cool 10 minutes more before serving.