Preheat the oven to 350ºF. Lightly butter twelve 4-ounce ramekins.
In a medium-size sauté pan, heat the olive oil over medium heat. Add the mushrooms and cook, stirring, for 2 minutes, then season with salt and pepper. Add the yellow onions and garlic, and cook, stirring, for 1 minute. Remove from the heat, stir in the herbs, and let cool.
In a large mixing bowl, combine the egg yolks and heavy cream and whisk to blend. Stir in the cheese, nutmeg, and the mushroom mixture. Spoon the mixture into the ramekins. Place the ramekins in a baking pan and fill the pan with enough water to come halfway up the sides of the ramekins. Bake until the flans set, 50 to 55 minutes.
In a medium-size sauté pan, fry the bacon until crispy, about 10 minutes. Transfer the bacon and 1/4 cup of the bacon grease to a large mixing bowl. Add the vinegar, mix, and season with salt and pepper. Add the spinach and red onions and toss to coat evenly.
To serve, gently loosen the mushroom flans with a knife and invert each onto a plate. Mound a small amount of the spinach salad in the center of each flan and serve.
Yield: 12 servings