Preheat your grill to medium high heat.
Add the potatoes to a a pot and cover with water, bring to a boil and cook for 10-12 minutes, just until tender. Immediately remove them from the water and run under cold water until cooled to stop the cooking process.
16 small new potatoes
Bring another pot of water to a boil then add the onions and cook for 5 minutes. Strain the onions, and once cooled to the touch use a tea towel to rub the skins off. Discard skins.
12 cipollini onions
Thread the steak cubes, cooled potatoes, cooled onions, and mushrooms onto skewers- alternating as you go.
2 lbs beef sirloin steak, 16 cremini mushrooms
Add the oil to a shallow bowl and turn the skewers in the oil to completely coat. Then season them generously with salt and pepper.
3/4 cup extra virgin olive oil, salt & pepper
In a small bowl add the steak sauce and spicy mustard, whisk together until evenly combined. Brush this sauce evenly out over all of the prepared skewers. Reserve any remaining spicy mustard steak sauce for serving.
1/2 cup steak sauce, 1/4 cup spicy mustard
Grill the skewers for 12- 14 minutes, turning every couple of minutes, until the meat and veggies are tender.
Serve and enjoy, with the reserved spicy mustard steak sauce for dipping.