Heat a large nonstick skillet over medium-high heat. Arrange mushroom caps, stem sides up, in pan; cook 5 minutes, shaking pan occasionally. Turn mushrooms over; cook 5 minutes, shaking pan occasionally. Place mushrooms, stem sides down, on paper towels to drain and cool.
Place 4 tablespoons pepitas, parsley, and next 7 ingredients (through garlic) in a food processor; process until chopped. With processor on, slowly pour oil through food chute; process until well blended.
Fill each mushroom cap with about 1 teaspoon pesto; top evenly with remaining 2 tablespoons pepitas.
Made this for a New Year's Eve dinner and it was a big hit! I put in nearly double the amount of crushed red pepper called for in the recipe, which boosted the "kick" factor considerably. Delicious and light. Great mix of flavors in a fairly simple recipe. A good way to kick off a big meal.
All of the women in our group thought these were an amazing appetizer! I love stuffed mushrooms and these were a bit different than any I've ever had, very light and healthy without the cheese overload. The pepita seeds were a wonderful twist in the pesto. My husband liked them but said he expected more 'kick' which is to be expected of my cooking ;}