Preheat oven to 450 and line a small baking dish or baking sheet (with a lip) with heavy foil – this will make cleanup easier!
Place beet rounds into prepared baking pan and toss with olive oil and salt and pepper.
Place in oven and bake until cooked through, about 30 minutes.
Meanwhile, in small bowl, combine white wine vinegar, dijon mustard, garlic, thyme, salt and pepper and whisk to combine - while continually whisking, slowly drizzle in olive oil to combine and emulsify.
Taste dressing and adjust seasoning to your liking.
When beets are done, remove from oven and place foil in bowl and into fridge to cool.
In a large bowl, combine the arugula, tomatoes and walnuts. Drizzle the dressing over it and toss.
Once beets are cooled, arrange the salad mix on the platter, put the beets on top, and sprinkle with feta.