Wash your mushrooms using a damp paper towel and then set them, cavity up, on a baking sheet. Remove the stems and chop them up finely. Set them aside.
Melt 3 oz. unsalted butter, or olive oil, in a skillet over medium-low heat and add in ¼ yellow onion minced. Saute for about 10 minutes until they become translucent.
Turn up your heat to high and add your minced garlic cloves and saute for no more than 1 minute.
Add your sherry vinegar, can of minced clams with juice, ½ tsp of oregano, ½ tsp of basil, ½ tsp of parsley and ⅛ tsp salt. Let simmer over high heat for 5 minutes.
Add ½ cup of panko breadcrumbs and mix until they absorb the clam juice. Turn off heat.There should be no more liquid on the bottom of your pan. If there is, keep the heat on until all the liquid is fully absorbed/evaporated.
When mixture is cool enough, stuff each of your mushrooms so they are nice and high. I like to use a mini ice cream scooper to get the perfectly round mound.
Sprinkle remaining panko breadcrumbs on top of each mushroom. If they do not stick, try to flatten the top of the filling a bit to create a flatter surface.
Bake for 8-9 minutes. If breadcrumbs are not browned yet, broil for 1-3 minutes. Garnish with fresh parsley and serve hot!