In a large skillet, melt the butter over medium-low heat. Add onions and let them cook, stirring occasionally, until they are golden brown and caramelized, about 45 minutes. Keep it low and slow. Trust me, it's worth the wait.*
In the meantime, make the turkey burgers. In a bowl, add ground turkey, Worcestershire sauce, tomato paste, smoked paprika, garlic powder, oregano, cumin, salt and pepper. Mix together until well combined.
Form ground turkey into 4 burger patties and place them in the fridge for 10 minutes to firm up a bit.
In a hot skillet, add olive oil. Cook turkey patties on medium-high heat for 7 minutes on each side, or until cooked through. Transfer to a plate.
To assemble, add mashed avocado on toasted buns, tomato slice, turkey burger, then top with caramelized onions. Serve and enjoy!
*Caramelized onions can be kept in an airtight container in the fridge for up to 1 week.