I first tasted their house dressing 10 years ago and have worked on cracking it off and on since then and think I finally hit it this year. Full disclosure, since I can not personally verify this is how its made at TU, I will only say it is "inspired" by the house dressing at TU but my friends couldn't tell which one was the real stuff from TU and which one was mine in blind taste tests, some of them were even preferring mine to the real stuff from TU in our blind taste tests...

3/4 cup Hellman’s/Best Foods Real Mayonnaise
4 tablespoons plus 1 teaspoon Kosciusko Spicy Brown Mustard
3 tablespoons HP Fruity Steak Sauce
2 teaspoon HP Steak Sauce
1/3 teaspoon Worcestershire sauce
1/4 teaspoon water, room temperature

Most important is to use these exact brands for first 4 ingredients. I tried A1 sauce but its too smoky, deep to match the real stuff. Also, you'll want to find the HP Sauce manufactured for British consumers and not the version manufactured for American consumers, surprisingly there is a difference! Any European or British food specialty store should carry the British version.