1 cup blue cheese, crumbled
1/4 cup mayonnaise
2 teaspoons Dijon mustard
1 1/2 pounds 80% lean ground beef
1 teaspoon salt, plus more to taste
1 teaspoon ground black pepper, plus more to taste
4 hamburger rolls, split
2 cups mixed greens, loosely packed
2 thin slices red onion , separated into rings
16-20 basil leaves