Preparation:
Preheat oven to 450°F with rack in middle.

Trim beets, discarding stems and greens (or reserve for another use), then wrap together in foil and roast until tender, about 1 1/4 hours. Let beets stand in foil at least 15 minutes, then remove from foil and cool. Remove skins and cut beets into 1/3-inch dice.

Whisk together shallots, vinegar, lemon juice, sugar, 1/2 teaspoon salt, and 1/4 teaspoon pepper, then whisk in oil. Reserve 2 tablespoon vinaigrette for watercress and toss remainder with beets. Let marinate at room temperature 30 minutes.

To serve:
Drain beets in a sieve set over a bowl and set beet vinaigrette aside.

Put pastry ring in center of a salad plate and gently press about 2 1/2 tablespoons fish into bottom of ring, then put about 1/4 cup beets over fish. Repeat on 7 more plates.

Toss watercress with reserved 2 tablespoons vinaigrette and mound over beets (a small handful per plate). Drizzle a little beet vinaigrette around each plate.