Kick'n Up Tuna

Tuna, a universally loved fish, but with many fish, I can't eat it plain. Well, I can, but that is just so boring. Grilled or seared tuna is always wonderful, and sometimes a squeeze of lemon can conjure Sicily, but mostly I need something more.
One combination I sometimes add to tuna is mashed potatoes. This may sound counter intuitive, but it is one direction to go to make a hearty meal. But two plain ingredients really need help. Many like the sound of wasabi potatoes, and that is a fun accompaniment to seared tuna.
Someday, I will share with you a recipe for wasabi potatoes and an herb sauce, but today I have something different in mind. Instead of kick'n up the potatoes, I propose a soy-mustard sauce with some zing and the potatoes to balance that fire. In the time you cook your potatoes, you can mix up your sauce and sear your tuna for a quick, tasty and interesting meal.  Serves 2.

Ingredients:
2 potatoes
1 tbsp ground mustard
1 tbsp hot water
1 tbsp rice vinegar
¼ cup soy sauce
2 tuna steaks
salt/pepper
2 tbsp canola oil
2 tbsp butter
¼ cup milk

Directions:
Peel and quarter potatoes, place in pot and add enough salted water to cover and bring to a boil. Cook until tender, about 15 minutes.
Meanwhile, mix mustard and hot water together. Whisk in vinegar and soy sauce. Set aside.
With a few minutes left in the potato cooking time, heat butter in a small sauce pan (or microwave in a bowl) until melted. Add milk and heat until warm. When potatoes are fork tender, drain. Put potatoes through a ricer, add warmed milk and butter and stir briefly. Riced potatoes will mash quickly. Add additional milk if needed until potatoes are especially smooth. Salt and pepper to taste. Cover to keep warm.
Season tuna steaks with salt and pepper. Heat oil in a large skillet over medium high heat until smoking. Add tuna, sear about 3 minutes. Turn tuna and sear the other side an additional few minutes. Outside should be seared golden while inside remains rare. Plate mashed potatoes with tuna on top, and drizzle soy-mustard sauce on top and around plate. Serve.