In a medium size skillet, melt butter and sauté diced onion with rosemary sprig over medium heat until soft and translucent, about 4 minutes.
Add garlic and cook until fragrant, about 2 minutes.
Remove the rosemary leaves from the sprig and discard the stem. Add the rosemary leaves, cooked onions, garlic, and butter from the pan to a food processor on top of fresh minced parsley (reserve a pinch for garnishing), walnuts, cream cheese, salt and pepper.
Pulse in food processor until combined but still chunky
Fold in breadcrumbs, Parmesan cheese, and minced sun dried tomatoes.
Clean mushrooms with a dry towel to remove any dirt. Remove stems and use a spoon to scoop out the gills of the mushrooms. Discard the gills and stuff mushrooms gently with a spoon so they are overflowing.
Drizzle with reserved sun dried tomato oil and rub the bottom of each mushroom in any oil that is leftover or has dripped onto your baking sheet.*
Bake the mushrooms in 400 degree oven on a lightly greased, parchment-lined baking sheet for 15 minutes
When done, simply garnish with the reserved minced parsley and a crank of salt and pepper on top! Enjoy.
*If you did not buy the kind of sun dried tomatoes that are stored in oil, you can rehydrate dehydrated sun dried tomatoes by soaking them in water on your countertop until they are soft and pliable. You can also soak them in olive oil until the same consistency is achieved! If you do not have sun dried tomato oil to drizzle on top, regular olive oil will do just fine.