1. Preheat oven to 400º. Place beets on a lightly greased 8-inch square glass baking dish. (If using both red and golden beets, separate on different sides of pan to prevent red from bleeding onto golden.) Pour 2 tablespoons balsamic vinegar and next 4 ingredients over beets; season with salt and pepper. Cover tightly with foil. Bake at 400° for 40 to 45 minutes or until tender. Uncover and let cool to room temperature, reserving beet juice. Peel beets (see tips, below), and slice into 1/4-inch rounds.
2. Separate leaves of endive, and tear radicchio into large pieces; arrange on a large platter or shallow bowl. Layer beets, oranges, fennel, and onion.
3. Whisk mustard and remaining 2 tablespoons balsamic vinegar into reserved beet juice; season with salt and pepper. Serve with salad. Garnish, if desired.
Sara's tips for cooking beets:
You can use different varieties of beets interchangeably. Whole baby beets are nice in a salad. I also like yellow beets for both their mild flavor and beautiful color.
Roast beets for the best flavor.
Peel beets by hand after they're cooked. The peel slips off easily, but it can be messy--wear gloves and an apron, and rub skin off with a paper towel.
Don't rinse peeled beets under running water. You'll wash away the wonderful flavor you just cooked into them.
Lightly rub a pumice stone over your soapy fingers if beets stain your hands.