**Instructions:**
1. **Make the Secret Sauce:**
In a small bowl, combine mayonnaise, ketchup, Dijon mustard, pickle relish, garlic powder, onion powder, and a pinch of salt and pepper. Mix well and set aside.
2. **Cook the Onions:**
Heat a skillet over medium heat with a little butter or oil. Add the sliced onions and cook for about 10-15 minutes, stirring occasionally, until they are golden and caramelized. Set aside.
3. **Make the Patties:**
Divide the ground beef into 4 equal portions and shape into thin patties slightly larger than your bread slices. Season both sides with salt and pepper. In the same skillet, cook the patties over medium-high heat for 3-4 minutes per side, or until cooked through. Remove and set aside.
4. **Assemble the Patty Melts:**
Spread butter on one side of each bread slice. On the unbuttered side, spread some secret sauce. Place a cooked patty on top of the sauce, add a slice of cheese, caramelized onions, and another slice of bread (buttered side facing out).
5. **Grill the Sandwiches:**
Heat a large skillet or griddle over medium heat. Grill the patty melts for 2-3 minutes per side, or until the bread is golden brown and crispy, and the cheese is melted.
6. **Serve:**
Slice the patty melts in half and serve hot with extra secret sauce on the side for dipping.
