Place potatoes into a pot of cold, salted water and bring to a boil. Reduce heat and cook until fork tender. Remove from heat, drain and set aside to cool. When cooled, cut into halves.
Season steak with salt and pepper to taste. Grill to medium rare. Allow to cool, then slice thinly.
Lightly brush potatoes with olive oil. Grill on BBQ over low heat until grill marks appear, about 7 minutes. Remove from heat and set aside.
To make blue cheese dressing: Mix all ingredients together.
In a large bowl, toss arugula and baby spinach with olive oil and sprinkle with salt. Add 3-4 tbsp of blue cheese dressing and lightly toss. Add grilled potatoes, steak, onion and garlic and toss again. Transfer to plates and add blue cheese.