This dressing is similar to the one at Cafe Rio. I could seriously drink the stuff.
You will need:
1 packet TRADITIONAL Hidden Valley Ranch dressing mix (not the one marked Buttermilk)
1 cup Mayonnaise
1/3 cup buttermilk
2 tomatillos (remove husk, cut in half)
1/2 bunch fresh cilantro
1 Clove garlic
1/2 jalapeno, seeded
Juice of 1 lime
Place all of the above ingredients in a blender and blend until smooth. If you like it spicey, add the whole jalapeno, seeds removed.
If it's too thick, add more buttermilk.