Preheat the oven to 400 degrees F (200 degrees C). Combine cream cheese, onion, and egg yolks in a large bowl; beat with an electric mixer until blended. Spread cream cheese mixture evenly on bread and lightly sprinkle with paprika. Place on baking sheets. Bake in the preheated oven until cheese puffs start to slightly brown, 10 to 15 minutes.