Preheat oven to 375 degrees. Line a rimmed baking sheet with nonstick foil. Place mushroom caps on prepared baking sheet.
24-36 whole white mushrooms
Chop mushroom stems. Heat butter over medium heat and add mushroom stems and onion. Saute for about 5 minutes or until onion begins to soften. Add panko crumbs and cook for 2 minutes. Remove from heat and let cool slightly.
2 Tablespoons butter, 1 onion, 1/2 cup panko crumbs
Combine spinach and artichokes in bowl. Set aside.
1 can quartered artichoke hearts, 10 ounce box frozen chopped spinach,
In a large bowl, combine cream cheese, sour cream, shredded cheeses and cooled mushroom mixture. Mix well. Stir in spinach mixture, Sriracha and salt.
8 ounces cream cheese, 1/3 cup sour cream, 1/4 cup shredded cheddar cheese, 3/4 cup shredded pepper jack cheese, 2 teaspoon Sriracha, 1 teaspoon salt
Spoon filling into mushroom caps. Bake at 375 degrees for 20 minutes.