Roasted Beet Salad with Flash-Pickled Radishes and Chard

Preheat oven to 400°F. Place beets in a small baking dish. Add just enough water to cover bottom of pan, drizzle beets with 2 Tbsp. oil, and sprinkle with 1/2 tsp. salt and 1/4 tsp. pepper. Cover pan tightly with aluminum foil (if foil is touching beets, line it with parchment paper) and bake beets until a knife can pierce them with little resistance, 55–70 minutes. Let cool slightly, then use a paper towel to slip skins off beets. Cut each beet into 1/4-inch-thick wedges and place in a medium bowl.
While beets bake, heat a small skillet over medium and toast walnuts until fragrant, shaking pan often, about 5 minutes. Transfer to a cutting board and coarsely chop, then transfer to a plate and drizzle with 1 Tbsp. oil; toss to coat. Sprinkle with 1/2 tsp. salt.
Whisk lemon juice, mustard, 2 tsp. vinegar, 1/4 tsp. salt, and 1/4 tsp. pepper in a small bowl until well combined. Whisk in 2 Tbsp. oil in a slow stream. While beets are still warm, toss with half of the vinaigrette to coat. Season with 1 tsp. pepper and 1/4 tsp. salt.
Place chard stems and radishes into 2 separate medium bowls and cover each with 1/2 cup vinegar and a pinch of salt. Let rest 5 minutes, then transfer vegetables to paper towels to blot, discarding excess vinegar. Combine vegetables in one of the medium bowls and toss with remaining vinaigrette.
Divide beets among 4 plates and top with walnuts, pickled radishes and chard stems, goat cheese, and top with a few grinds of black pepper.
Beets can be roasted and refrigerated up to 3 days in advance. Vinaigrette can be made and refrigerated up to 5 days in advance.