I've checked a wide range of different Greek salad recipes, and I have yet to find any that use spinach greens. Many use romaine, which is perfectly okay. Some use iceberg (*shudder*). But, none use spinach. Why is that? Spinach is important to Greek cuisine (have you ever had a spanakopata - spinach pie?) And, in this application, it works perfectly.
I dried the (plum) tomatoes and made a Greek dressing last night. But, the beets are a thing I like to call a classic "chain reaction cooking decision". Here's the process: