"You don't need a charcoal grill to develop a nice crust on your burgers. Sear on cast iron instead for a similar result inside...."
INGREDIENTS
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1 1/2 pounds ground beef sirloin
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2 teaspoons Worcestershire sauce
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1 tablespoon snipped fresh oregano or 1 teaspoon dried oregano, crushed
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1/2 teaspoon kosher salt
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1/4 teaspoon freshly ground black pepper
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2 tablespoons olive oil
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1 1/2 cups thickly sliced button or cremini mushrooms
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2 small red onions, sliced
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1 red sweet pepper, stemmed, seeded, and cut into strips
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1/3 cup crumbled feta cheese
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4 kaiser rolls, split and toasted