Remove the stems from the portobello mushrooms and chop the stems. Set the whole mushroom caps aside.
In a medium heated frying pan add olive oil, onion, garlic, and mushroom stems. Saute until soft, about 3 minutes. Deglaze pan with white wine.
Pour in half-and-half and mix well. Add chopped spinach and cook until soft, about 5 minutes. Shake in grated Parmesan and breadcrumbs. Sauce will thicken when heated through, another few minutes. Remove from heat.
On a baking sheet, or large pan with a grate, arrange mushrooms, cavity-side up. Spoon in enough spinach mixture to fill the portobello mushroom cap. Stack on a couple spoonfuls of crab. Top with mozzarella cheese. Bake for 25 to 30 minutes, checking occasionally to make sure the cheese doesn't burn. Remove mushrooms from the oven and serve warm.