Preheat oven to 425 degrees.
Loosely wrap beets in parchment-lined foil.
Place on a rimmed baking sheet and bake until easily pierced with a knife, about 1 hour.
Let cool slightly; remove skins.
Season pork generously with salt and pepper.
Heat vegetable oil in a large ovenproof skillet over medium-high heat.
Sear pork on all sides until golden, about 6 minutes.
Transfer skillet to oven and roast until pork registers 150 degrees on an instant-read thermometer, 8 to 10 minutes.
Remove from pan and let rest for 10 minutes.3.
Meanwhile, whisk mustard and vinegar in a bowl.
Gradually whisk in 6 tablespoons olive oil until emulsified.
Season to taste; set aside.4.
In a large skillet, heat remaining tablespoon of olive oil over medium heat.
Add garlic and cook until fragrant, about 1 minute.
Add beet greens, season with salt and pepper, and cook, stirring occasionally, until wilted and tender, 2 to 4 minutes.
Remove greens from heat.
Add reserved sliced beets and half of vinaigrette, and toss to combine.5.
Slice pork and drizzle with remaining vinaigrette.
Serve with sautéed greens and sliced beets.