Grilled Potato and Onion Salad with Blue Cheese and Bacon

1 pound sliced meaty bacon
3 red onions, sliced 1/2 inch thick
1/4 cup extra-virgin olive oil
Kosher salt and freshly ground black pepper
3 1/2 pounds small new potatoes (about 1 1/2 inches in diameter), scrubbed 1/2 cup mayonnaise
1/2 pound blue cheese, such as Gorgonzola, Roquefort or Maytag Blue, crumbled
Preheat the oven to 400°. Spread the bacon slices in a single layer on a baking sheet. Bake until they are brown and crisp, about 15 minutes. Transfer the bacon slices to paper towel lined plates to drain. Crumble into 1/2-inch pieces. Light a grill. In a large bowl, toss the red onion slices with 1 tablespoon of the olive oil. Season with salt and pepper. Grill the onions over moderately high heat, turning once, until lightly charred and tender, about 5 minutes. Cut the grilled onions into 1/2-inch pieces. In a large pot of salted cold water, bring the potatoes to a boil. Simmer until they are beginning to get tender, about 5 minutes. Drain the potatoes and cut them in half. In a large bowl, toss the potatoes with the remaining 3 tablespoons of olive oil and season with salt and pepper. Grill the potatoes over moderately high heat, turning once, until they are tender and browned, about 5 minutes. In a large bowl, mix the mayonnaise with the blue cheese. Add the bacon, onions and potatoes and toss to coat. Transfer the salad to a clean bowl and serve.