Preheat the oven to 375 degrees F. Coat a baking sheet with cooking spray. Remove the stems from the mushrooms and coarsely chop them. Scrape the gills from the underside of the mushroom caps using a spoon; discard.
Heat a large skillet over medium heat. Add the butter, half of the scallions, the poblano and garlic and cook, stirring, until the poblano is soft, about 4 minutes. Add the mushroom stems and cook, stirring occasionally, until tender, about 3 minutes. Stir in 1/4 cup wheat germ and cook 1 minute. Remove from the heat; stir in the parsley and season with salt and pepper.
Stuff the mushroom caps with one-quarter each of the mozzarella and poblano mixture and arrange on the prepared baking sheet. Sprinkle each with 1 tablespoon wheat germ and generously spray with cooking spray. Bake until the mushrooms are tender and the wheat germ is golden, about 20 minutes. Top with the sour cream, jalapeno and the remaining scallions.