Brush any dirt from the tops of the portobellos, remove the stems and reserve, then gently scrape the gills out with a spoon.
Place the mushrooms (tops down) on a baking sheet and brush lightly with olive oil. Season with a little salt and pepper and bake for 6 minutes. Remove from oven and blot away the excess liquid that has pooled inside the mushroom cap with paper towels. Set aside.
Fry the bacon in a large skillet until crisp. Drain on paper towels, chop roughly and set aside. Discard all but 1 tablespoon of the fat remaining in the pan.
Roughly chop the reserved mushroom stems and add them to the pan along with the onion. Sauté over medium-high heat until the onion is soft and translucent, 2 minutes. Add the spinach and cook until slightly wilted, 1/2 minute. Sprinkle with sugar, add the vinegar, and continue cooking just long enough for the spinach to wilt completely, about 1 minute more. Add the bacon, cream and a few grinds of black pepper. Combine thoroughly.
Divide the spinach mixture evenly among the mushroom caps. Use tongs for this task and allow the excess liquid to drain from the spinach mixture before adding it to the mushrooms. Top each mushroom with a slice of cheese and return them to the oven for about 6 minutes, or until the cheese is melted and the mushrooms are heated through.
Tips for Making This Recipe
We like to precook the mushrooms for a few minutes before filling to eliminate some of the extra liquid they contain. This ensures that the mushrooms are tender and the filling isn't swimming in liquid.