Soak leeks in cold water for 5 minutes; lift out and drain. Repeat until no grit remains on bottom of bowl. Set aside.
Heat oil in a large skillet over medium-high
heat. Add leeks and salt and saute
until leeks are tender, about 2 minutes. Stir
in asparagus, then water. Simmer covered
for 2 minutes. Add snap peas and radishes,
cover, and simmer for 2 minutes more.
Stir in mustard until well combined, then swirl in butter and herbs. Serve immediately.