Slope disappearing

I have wood steps built next to my garage, but up over a slope. The slope is washing away under the steps.  How do I keep the soil under the steps from washing away again once I fill it in? 

That's a tough question without seeing the area or knowing how steep the slope is, but here are some ideas.

Fill it in with something that plants will like, compost would be perfect. Now plant something to hold the soil. The first thing I would think of is a quick growing grass, but you could also use a ground cover, daylilies or other perennials.

It might be worth it to have a landscaper look at it, because there is a structual issue concerning the stairs.