Preheat the oven to 400 degrees F (200 degrees C).
Peel onions and cut off ends. Place each onion on large sheet of aluminum foil.
Cut 6 to 8 slices in each onion from the top down about 3/4 of the way toward the bottom; don't cut all the way through. Sprinkle 1/2 of the crumbled bouillon cube over each onion, then place 1 tablespoon butter on top of each. Season with paprika, salt, and pepper. Wrap aluminum foil around each onion to form a pouch.
Roast in the preheated oven to desired tenderness, 30 to 60 minutes.
Be careful when opening foil pouches as steam will be hot. Drizzle cooking liquid over onions once served.