Heat oven to 400°F (200°C). Put beets in a small baking dish with the water. Cover tightly with a lid or foil. Roast 45–60 minutes, until a small knife slips easily into the center. Large beets may need longer.

Let beets cool until safe to handle, about 20 minutes. Rub off skins with a paper towel or peel carefully, then cut into wedges. Protect the counter because beet juice stains.

Cut a thin slice from each end of the oranges. Remove peel and white pith with a knife, then cut segments or bite-size pieces, catching any juice in a bowl. Remove seeds.

Whisk 1 tablespoon collected orange juice with oil, vinegar, mustard, salt and pepper. If there is little juice, use 1 tablespoon water instead.

Lightly toss arugula with half the dressing. Arrange beets and oranges over the leaves, add walnuts and goat cheese, then spoon over remaining dressing. Avoid vigorously tossing everything together if you want the cheese to stay pale.