Preheat oven to 400°. Trim tops from beets; set aside half of tops (reserve remainder for another use). Scrub beets, pat dry, and slice ¼” thick. Toss in a large bowl with 4 Tbsp. oil; season with salt and pepper.

Scatter marjoram sprigs across a rimmed baking sheet and arrange beets on top in as even a layer as possible (some overlap is okay). Roast, tossing occasionally, until beets are tender, 15–20 minutes. Remove from oven and toss in reserved beet greens; roast just until slightly wilted, about 2 minutes. Let cool.

Meanwhile, whisk crème fraîche, yogurt, and 1 tsp. vinegar in a small bowl; season with salt; set aside.

Smash sesame seeds, 2 Tbsp. marjoram leaves, and a generous pinch of salt in a mortar and pestle to combine. (Alternatively, chop with a knife.) Transfer to a small bowl and mix in remaining 1 Tbsp. oil; set sesame salt aside.

Drizzle roasted beets and greens with remaining 1 Tbsp. vinegar; season with salt and pepper. Spoon reserved crème fraîche mixture between 2 platters and arrange beets and greens on top; sprinkle with sesame salt.

Do Ahead: Beets (without greens) can be roasted 1 day ahead. Let cool; cover and chill. Bring to room temperature before using; sauté greens separately. Sesame salt can be made 2 days ahead; cover and chill.