Recipe: Spinach and Blue Cheese Burger

Name: Recipe: Spinach and Blue Cheese Burger
Ingredients: 2 tablespoons vegetable oil, divided; 2 medium yellow onions, thinly sliced; Kosher salt; Freshly ground black pepper; 2 large hamburger buns, split; 2 (1/3-pound) hamburger patties; 3 ounces Roquefort or blue cheese; 1 tablespoon crème fraiche or plain Greek yogurt; 2 handfuls baby spinach