Heat oven to 400 degrees. Arrange greens and beets in a small baking dish. Drizzle with oil, season with salt and pepper, and toss to coat.
In a small bowl, whisk oil and mustard. Season halibut with salt and pepper and place on top of greens. Spoon over mustard sauce and cover with parchment-lined foil. Bake until greens are wilted and fish is cooked through, about 20 minutes. Squeeze with lemon.