Preheat oven to 450 degrees F 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 cool completely before placing in the fridge to chill.
In a large bowl, combine the salad mi and tomatoes. 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 and walnuts.
You can use a mix of red and golden beets. They will add slightly different flavors, but very pretty colors!
You could use goat cheese instead of feta if you prefer.
Make a balsamic vinaigrette instead. Replace the white wine vinegar with balsamic vinegar. Add a little maple syrup or honey if you like your dressing sweet.
You can roast the beets whole. Wrap the beets in foil, and cook as directed and maybe add 15 minutes to cooking time. Remove from the oven and when they’re cool enough to handle, slice them (and chill them).