Remove stems from the caps from cleaned mushrooms. Set caps aside.

Place stems in a food processor.

Add drained clams, onion and carrot. Process until finely chopped.

Add butter about 1/4 cup butter to a large saucepan. Place pan over medium heat and melt butter.

When butter has melted and is hot, add garlic. Cook, stirring, about 1 minute.

Add mushroom/clam mixture from the food processor to saucepan. Sauté until tender and fragrant, about 5 to 7 minutes.

In a medium bowl add Parmesan, Romano and mozzarella cheeses. Mix well. Remove about 1/3 and set aside in a small bowl.

To cheeses in medium bowl, add sautéed mushroom/clam mixture, 1 cup bread crumbs and egg. Mix well. Mixture should pack well and not be wet or crumbly. Add more breadcrumbs to get the right consistency.

Preheat oven to 350°F.

Stuff mushrooms caps with prepared mixture.

Place stuffed mushrooms, stuffing up (like a small cup) into a 13 x 9-inch baking dish.

Sprinkle remaining cheese on top.

Cut remaining 1/4 cup butter into pats and place over top of cheese.

Place in preheated oven. Cook until mushrooms are soft, about 15 to 20 minutes.

Turn the oven to broil. Broil about 3 - 5 minute to brown the tops. Watch closely, so you do not burn the tops.

Remove from oven and serve immediately, hot.