Preheat the grill. Season the corn with a drizzle of olive oil, salt and pepper. Place the corn on the grill (you can also use either an open gas burner or roast the corn in the oven) Grill the corn for 10 minutes, turning the corn every 2 minutes. In a saute pan, heat the olive oil. Add the pancetta and render until crispy, about 6 minutes. Add the onions. Season with salt and pepper. Continue to cook for 6 minutes, stirring occasionally. Remove the corn from the grill. Scrape the kernels from the cob. Add the corn to the pancetta and onion mixture. Continue to saute for 1 minute. Stir in the garlic and vinegar. Continue to cook for 1 minute. Remove from the heat. Place the spinach in a large mixing bowl. Season with salt and pepper. Add the dressing to the bowl and toss the salad completely. To serve, mound the greens in the center of each plate. Crumble the cheese over the top of the salad and serve.