An alteration of a recipe from Joy of Cooking. This is an incredibly flavorful warm or room temperature side dish. Not exactly a salad. It is versatile and I find that it compliments roasts as well as makes nice addition to summer salad buffet.
Toss diced potatoes with 3 tablespoons of olive oil and garlic. Salt generously and sprinkle with rosemary.
Arrange in a single layer on a rimmed cookie sheet and roast about 15-20 minutes.
Meanwhile, toss diced beets with remaining 3 tablespoons olive oil and onion, salt generously.
Arrange in a single layer on a rimmed cookie sheet and put in oven with potatoes until potatoes begin to brown.
While potatoes and beets are finishing in the oven, mix together vinaigrette: vinegar, mustard, 1/3 cup olive oil, salt and pepper.
Remove potatoes and beets from pan and add 'pan drippings' to vinaigrette.
Drizzle warm potatoes and beets with vinaigrette and either arrange veggies side-by-side on a platter (to minimize the color bleed) or for a more casual meal, gently toss them together in the same serving dish.