For the pancakes: Dissolve yeast and sugar in 2⁄3 cup warm water in a small bowl and set aside until foamy, about 10 minutes. Combine flour and semolina in a large bowl, then add milk and 1 cup water and beat on medium speed with an electric mixer until smooth, 2-3 minutes. Add yeast mixture and continue beating until batter is smooth, about 1 minute. Combine baking soda and 1 1⁄2 tsp. water in a small bowl and beat into batter on medium speed. Cover bowl with plastic wrap and set aside in a warm spot until batter is foamy, about 1 hour.
For the filling: Combine walnuts, sugar, and cinnamon in a medium bowl and set aside.
For the syrup: Put sugar, lemon juice, and 1 1⁄2 cups water into a medium saucepan and bring to a boil over medium-high heat. Cook, stirring often, until sugar dissolves, 2-3 minutes. Keep syrup warm over lowest heat.
Preheat oven to 350°. Heat a medium cast-iron or other heavy skillet over medium heat until hot but not smoking. Brush skillet with a thin layer of oil. Pour 1⁄4 cup of the batter into skillet and swirl skillet to spread batter out to a 5"-wide pancake. Cook, undisturbed, until bottom is browned and top is covered with bubbles and no longer moist, 1-2 minutes. Do not flip pancake. Transfer pancake to a clean surface and cover with a clean dry dish towel. Repeat process with the remaining batter to make 12 pancakes in all, brushing skillet with more oil as needed.
To serve: Put one pancake, browned side down, on a clean surface. Spread 2 tbsp. of the filling down center of pancake, fold pancake in half, and press seams shut to enclose filling completely. Repeat process with the remaining pancakes and filling. Brush both sides of filled pancakes with melted butter and transfer to a baking sheet. Bake until warmed through, 5-6 minutes. Dunk pancakes, one at a time into the warm syrup. Serve with remaining syrup on the side.