Preheat the oven to 350°F.
Give the mushrooms a good wash, removing any dirt that might be stuck to the caps or stems. Trim the stems and set them aside. With a small spoon or, I liked to use a melon scoop, carefully scoop a hole into the mushroom cap. Set what you scoop out aside with the stems. When done, set your caps aside.
Roughly chop the mushroom stems and extra bits. Heat the oil in a small saucepan over moderate heat, and add the mushroom bits, minced garlic, and a pinch of kosher salt. Sauté for about 3-5 minutes, or until the garlic is fragrant and the mushroom bits are tender. Set aside to cool.
In the meantime, transfer the sardine fillets to a bowl and flake the fillets with a fork. Add in the diced jalapeños and cream cheese, and once the mushrooms and garlic have cooled, add them as well. Stir and fold the mixture until everything is well incorporated.
Lightly grease a baking sheet and place the mushroom caps on top. Spoon a generous amount of the mixture into the mushroom caps. Don't worry if the mixture is brimming over—the more stuffing, the better. When done, set aside.
Melt 1 TBSP of butter in a small bowl, then add the bread crumbs. Stir the breadcrumbs into the melted butter until all of the breadcrumbs become dark, buttery, and super crumbly. Finally, sprinkle a generous amount of breadcrumb topping over each mushroom cap.
Place the baking sheet into the oven and bake the mushrooms for about 12-15 minutes, or until the mushrooms are tender. Sprinkle with some freshly chopped parsley and serve.