1. Prepare a medium-hot fire in a charcoal grill or preheat a gas grill on high for 15 minutes. If using a gas grill, leave the primary burner on high and adjust the other(s) to maintain grill temperature at about 350 degrees F. If using charcoal, when the charcoal is covered with gray ash, push it to one side of grill, position grate, cover, and heat grate for 5 minutes. Scrape grate clean with a grill brush. Lightly oil grill grate with vegetable oil.
2. While grill heats, prepare spinach according to package instructions. Let cool.
3. Heat 1 Tbsp. of the olive oil in a medium skillet over medium heat until it shimmers. Add onion and cook, stirring, until softened, about 4 minutes. Add garlic and red pepper flakes and cook, stirring, until fragrant, about 40 seconds. Set aside off heat to cool briefly.
4. When spinach is cool, add to a sieve and press down to release as much liquid as possible; discard the liquid. Transfer spinach to a medium bowl, break it up with your fingers (there should be about 11/2 to 2 cups). Add onion mixture, goat cheese, Parmesan, salt, and pepper and stir until well combined and uniform.
5. Scrape the gills from the undersides of the mushroom caps with a teaspoon. Spray both sides of caps with cooking spray, and divide spinach mixture among them. Position mushrooms on cool side of grill (opposite the fire or burner), close cover (for charcoal grill, make sure cover vents are halfway open and positioned over mushrooms) and roast until mushrooms are tender and filling is heated through, about 25 minutes. Serve at once, drizzled with additional olive oil if desired.
Source: Hannaford fresh Magazine, May - June 2014