Add a teaspoon of canola oil to a medium nonstick frying pan or skillet, and let it get nice and hot over medium-high heat. Meanwhile, salt and pepper both sides of each steak (heavier on the pepper than the salt). Add steaks to the pan, and cook to desired doneness (about 5-6 minutes per side for medium-well). Use a fork to remove steaks to a plate (keep the steak juices in the pan for use later).
Add the remaining teaspoon of oil to a large, nonstick skillet and sauté the onion slices over medium-high heat until golden (about 5 minutes). Add the mushroom slices, 1/2 cup broth or water, and all the juices from the steak pan. Continue to cook the onions and mushrooms, stirring often, for about 5 minutes. Remove from heat.
Put light mayonnaise and horseradish in a custard cup and stir to blend well.
Place the bottoms of your sandwich rolls on a serving plate. Spread horseradish sauce on top of each. Top each with steak slices and the onion/mushroom mixture, then place tops on the sandwiches.