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Preheat grill to medium-high heat.
In a small skillet, heat olive oil and butter over medium-high heat. Add the onion, peppers, salt and fajita seasoning; saute for 4-6 minutes, or until slightly soft.
Meanwhile, in a large bowl, gently combine the beef, fajita seasoning and Worcestershire sauce. Divide ground beef mixture into four equal portions and form into patties.
Let burgers rest for 4-5 minutes then transfer to preheated grill. Cook burgers for 4-5 minutes per side, or until cooked through. During the last 1-2 minutes of cooking top burgers with cheese and allow to melt.
To serve; top each burger with the pepper and onion mixture (reheated, if needed) and sliced tomatoes. Serve on lightly toasted (if desired) buns with Creamy Avocado Ranch Spread.