To make dressing: In blender, combine yogurt, lime juice, milk, chives, garlic, salt, and pepper. Blend on low until smooth consistency forms and chives are completely incorporated.
Transfer dressing to jar with tight-fitting lid and refrigerate until serving.
Heat grill to medium. To prepare steak: Combine kosher salt, black pepper, and granulated garlic to create a rub. Sprinkle half of seasoning mix over one side of steak, pressing it into meat. Repeat with remaining seasoning on opposite side of steak.
Grill steak over direct medium heat to desired level of doneness, approximately 4-5 minutes per side for medium pink center. Remove steak from grill and let rest 7-10 minutes on cutting board.
To make salad: Toss spinach and romaine on large platter. Scatter red and yellow peppers, avocado, and onion over greens. Slice grilled sirloin thinly against grain. Arrange meat slices along center of salad. Drizzle dressing over salad just prior to serving.