Preheat the oven to 350 degrees F. Lightly butter 12 (4-ounce) ramekins. In a saute pan, heat the olive oil. When the oil is hot, add the mushrooms and saute for 2 minutes. Season the mushrooms with salt and pepper. Add the onions and tablespoon of garlic.
Saute for 1 minute. Remove from the heat, stir in the herbs and cool. In a mixing bowl, whisk the eggs and cream together. Stir in the cheese, nutmeg, and mushroom mixture. Spoon the mushroom/cream mixture into the ramekins. Place the ramekins in a baking pan and fill the pan with enough water to cover 1/2 of the ramekins. Place the pan in the oven and bake for 50 to 55 minutes or until the flan is set. In a saute pan, render the bacon until crispy, about 10 minutes. Turn the crispy bacon and bacon fat into a mixing bowl. Whisk in the balsamic vinegar. Season with salt and pepper. Toss the spinach and red onions with the vinaigrette. To serve, run a knife around each ramekin, invert each flan onto a plate. Mound a small amount of the spinach salad in the center of each flan.