Preheat the oven to 375°F. Rub the portobello caps with a little olive oil, sprinkle with salt and pepper and place, tops down, on a sheet pan.
In a large skillet over medium-high heat, sauté the onions and garlic in the oil, and cook for a few minutes until the onions become soft. Squeeze any excess liquid out of the thawed spinach, and add the Italian seasoning, salt, pepper and spinach to the onion and garlic mixture. Stir well, and sauté for another few minutes. Add the milk, almonds, and half the shredded cheese; mix well. Taste the filling for salt and pepper.
Distribute the filling evenly among the mushroom caps and top with the remaining shredded cheese. Bake for 15-20 minutes until mushrooms are tender and cheese is melted and beginning to brown.
This classic appetizer is also delicious for dinner, with a side of buttered noodles or fettuccini and a drizzle of freshly-made pesto. Experiment with adding chopped sun-dried tomatoes, blue cheese or feta crumbles to the filling, or substituting chopped toasted walnuts or pine nuts for the sliced almonds.