Horseradish, Cheddar, and Caramelized Onion-Stuffed Burgers - Ellie Krieger

Ingredients

• 2 teaspoons olive oil
• 1 medium onion, sliced into half-moons
• 1 pound lean ground beef (90% lean or higher)
• 4 teaspoons prepared white horseradish, drained of excess liquid
• ½ cup lightly packed shredded extra-sharp cheddar cheese (2 ounces)
• ¼ teaspoon salt
• 1/8 teaspoon freshly ground black pepper
• Nonstick cooking spray
• 4 whole-wheat hamburger buns
• 4 medium leaves romaine lettuce, ribs removed
• 1 large tomato, sliced