Preheat the oven to 350 degrees F. Set out a 9x13-inch baking dish. Gently pop off the stems of the mushrooms. Discard the stems or save for later use. Then use a damp paper towel to brush off the dirt on the mushroom tops. Do rinse them in water.
In a medium bowl, mix the cream cheese, smoked cheddar cheese, diced red pepper, onion, and garlic together. Add 1/4 teaspoon salt and 1/4 teaspoon ground black pepper. Mix well.
Use a spoon to pack the cavity of each mushroom with cheese filling. It's ok if it pops over the top a little. (But not too much or they will look messy.)
Press each cheese-stuffed top in grated Parmesan cheese to create a crust. Then set them in the baking dish, cheese-side-up. (It's okay if they are tightly fitted in the baking dish, because they will shrink while baking.)
Bake for 30-35 minutes, until the tops are puffed and golden. Serve warm.
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Notes
Don't overfill! You do not have to use all the filling. Mushroom cavities vary, so there's a chance you might have some cheese filling leftover. Don't be tempted to pile the filling high, or you might end up with a big mess. This is even more likely if your oven temperature runs hot. Use the less-is-more mentality when filling the mushrooms with just a small dome of filling on top.This is a great party appetizer to prep a day or two ahead, then bake right before your event.