Roast beets: Preheat oven to 450°F with a rack in the upper third. Scrub and quarter beets; cut into ¼-inch thick slices. Place in the center of a piece of foil. Drizzle with oil and season with salt and pepper. Fold up sides to enclose, then place on a rimmed baking sheet. Roast on upper oven rack until beets are tender, 20–25 minutes. Remove from oven and carefully open (steam will be hot).

Cook quinoa: Meanwhile, place quinoa in a small saucepan along with ¾ cup water and a pinch of salt. Bring to a boil over high heat. Reduce heat to low, cover, and cook until water is absorbed and quinoa is tender, about 15 minutes. Set aside, covered, until step 6.

Marinate vegetables: Peel cucumber; halve lengthwise, scoop out the seeds, then thinly slice crosswise. Trim radishes, then thinly slice. In a medium bowl, whisk to combine 1 tablespoon vinegar and 2 tablespoons oil. Add vegetables to the bowl, and toss to combine. Season to taste with salt and pepper. Set aside to marinate until step 6.

Make sauce & prep salmon: In a small bowl, stir to combine all of the sour cream and horseradish; season to taste with salt and pepper. Pat salmon dry; season on both sides with salt and pepper. Heat 1 tablespoon oil in a medium nonstick skillet over medium-high until shimmering.

Cook salmon: Reduce heat to medium, then add salmon fillets to the skillet, skin side down. Press each fillet firmly in place for 10 seconds with the back of a spatula. Continue to cook, occasionally pressing gently on fillets, until skin is well browned and very crisp and salmon is almost cooked through, 5–7 minutes. Flip salmon; cook until it is just medium, 1–2 minutes more.

Finish & serve: Coarsely chop dill fronds and stems together. Stir half of the dill into bowl with marinated vegetables. Spoon quinoa into bowls, and top with salmon, marinated veggies, roasted beets, and a dollop of the horseradish cream; drizzle beets with any remaining marinade from vegetables. Garnish with remaining dill.