To make the dressing, combine all of the ingredients in a jar. Cover and shake well. Set aside.
Cook the bacon in the microwave for about 3-4 minutes or until crisp. Set aside to cool. Crumble the bacon when it is cool enough to handle.
Heat the oil in a nonstick frying pan over medium-high heat. Add the onions and let them cook until wilted.
Add the mushrooms to the pan with the onions. Reduce the heat to medium, and let the vegetables cook until the onions are golden brown and caramelized. Stir the vegetables occasionally.
When the onions and mushrooms are cooked, move them to the sides of the pan, and add the cheese in the middle. Turn the heat up a bit and let the cheese cook for about 3 minutes on each side. It should be just lightly golden brown and starting to melt.
While the cheese is grilling, add the spinach to a large mixing bowl and toss it with as much of the dressing as needed. You may have dressing leftover.
Remove the pan from the stove, and add the dressed spinach. Toss everything in the pan, or if preferred, add everything back to the mixing bowl and toss to combine with the dressing.
Garnish the warm salad with the crumbled bacon and sliced/chopped eggs.