In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, salt, and poppy seeds.
In a separate mixing bowl, combine the melted unsalted butter, vegetable oil, eggs, sour cream, vanilla extract, lemon zest, lemon juice, and milk.
Pour the wet ingredients into the dry ingredients.
Using a rubber spatula, gently fold the mixture until just combined. Do not overmix.
Cover the bowl with plastic wrap.
Allow the batter to rest at room temperature for 1 hour.
While the batter rests, prepare the lemon simple syrup.
Add the fresh lemon juice and granulated sugar to a small saucepan.
Place the saucepan over medium heat.
Bring the mixture to a boil.
Reduce the heat and simmer for 5 minutes, or until the sugar is fully dissolved.
While the syrup simmers, carefully remove the peel from 1 lemon using a paring knife. Avoid cutting deeply into the white pith.
Place the lemon peel in a heat-safe bowl.
Remove the lemon syrup from the heat.
Pour the hot syrup over the lemon peel.
Allow the syrup to cool at room temperature while the muffin batter continues to rest.
Near the end of the resting time, preheat the oven to 425°F.
Line a muffin tin with 11 paper liners.
Using a large 2 oz scoop, or approximately 1/4 cup, divide the batter among the prepared liners.
Fill each liner to the top and slightly above the rim.
Avoid stirring or deflating the batter while scooping.
Bake the muffins at 425°F for 8 minutes.
Without removing the muffins from the oven, reduce the oven temperature to 350°F.
Continue baking for 8 to 10 minutes, or until a toothpick inserted into the center of a muffin comes out clean with a few moist crumbs.
Remove the muffins from the oven.
While the muffins are still warm, brush the tops generously with the prepared lemon simple syrup.
Allow the muffins to cool in the pan for 5 minutes.
Transfer the muffins to a wire cooling rack.
Allow the muffins to cool for an additional 15 minutes.
Sprinkle with additional poppy seeds, if desired.
Serve warm or at room temperature.