Gently clean the Portobello mushrooms and remove the stems.
Heat a griddle with a few drizzles of olive oil and place the mushrooms on the griddle to grill for about 3-4 minutes on each side.
Remove the mushrooms and set aside.
Heat a frying pan with a drizzle of olive oil. Place the chopped garlic and the frozen creamed spinach. When the creamed spinach has melted add the lemon juice and the creamed cheese. Stir until the cream cheese is well incorporated into the spinach.
Spoon the mixture into the mushrooms evenly and top with fresh ground black pepper.
Heat the griddle with another drizzle of olive oil and place the stuffed mushrooms back on the griddle. Cover the mushrooms loosely and grill for 3-4 minutes.