Whisk egg and milk together in a medium bowl. Add cornmeal, flour, salt, baking powder, and marjoram, stirring to combine. Fold in chopped onions, jalapeño, and corn.
Heat 2 tbsp. oil in a large nonstick frying pan over medium heat. Scoop 1/4-cup portions of batter into pan. Cook, turning once, until puffed and browned, about 6 minutes total. Repeat with remaining batter, adding more oil between batches. Serve with sour cream and sliced onions.
*Buy in a well-stocked grocery store or natural-food store.
Note: Nutritional analysis is per 2-cake serving.