In a small pan grill the onion with the butter and soy sauce until onions are soft, about 3-5 minutes.
Remove the onions and set aside. Add in the ground beef and mustard and cook until fully cooked throughout. Set aside.
Prepare the sauce by combining the mayonnaise, ketchup, white vinegar, sugar, sweet relish, minced onion. Stir until fully combined, will be a smooth sauce. Set aside.
Cube the Velveeta and place into a microwave safe bowl. Heat for 5 minutes, stopping every minute and half to stir. As the cheese begins to melt and is able to be stirred, add in the milk to help thin it, continue heating until fully melted.
Add in the onions, ground beef, and sauce, stir until fully combined. Serve warm with chips!