Add the spinach and water to a large pan set over medium heat. Cook 3-4 minutes, or until the spinach is wilted. Stir in the bacon and lemon zest.
Add the cheese and cook, stirring constantly, until melted. Stir in the bread crumbs and season to taste with red pepper flakes.
Stuff the filling into the portobello caps. Grill over high heat 4-5 minutes, or until the mushrooms are cooked through.
Serve with hot sauce, to taste.