Preheat oven to 450 degrees. Combine potatoes, onion slices, 1 tablespoon oil, and 3/4 teaspoon salt on a rimmed baking sheet. Season with pepper, and toss. Spread mixture in a single layer (use a second sheet if necessary). Roast, stirring potatoes and scraping bottom of sheet about every 10 minutes, flipping halfway through, until potatoes are brown and crisp, 40 to 45 minutes.

Combine mustard and lemon zest and juice in a bowl. Heat remaining 2 teaspoons oil in a large straight-sided skillet over medium-high heat. Add garlic, and cook, stirring constantly, until golden brown, about 2 minutes. Add kale, and cook, stirring occasionally, until wilted, about 6 minutes. Add mustard-lemon mixture; toss to coat. Cook until heated through. Sprinkle with 1/4 teaspoon salt, and season with pepper. Toss with potatoes.