Remove stems from mushroom caps (reserving stems) and gently rinse both caps and stems separately. Finely dice mushroom stems and use enough to make ½ cup. Combine cream cheese, half the cheddar cheese, garlic powder, diced mushroom stems and onions in a medium bowl. Mix well.
Melt butter in a pan over medium heat and sautée mushroom caps, for about 5 minutes, and then repeat with second batch. Stuff caps with filling and arrange caps in a shallow baking dish and sprinkle with bread crumbs, remaining cheddar cheese and paprika. Broil until cheese is melted.