Place potatoes in a pot and cover with cold water. Bring to a boil. Reduce heat to low and simmer 10 minutes or until fork tender. Drain.
In a large bowl whisk together mustard, vinegar, garlic, salt and pepper. Slowly whisk in oil.
Add potatoes while warm, scallions, feta and jalapenos.
This is fabulous serve a little warm. It sits out well as there is no mayo to spoil.