In a large Dutch oven over medium heat, heat the olive oil; cook and stir onion and garlic until the onion is translucent, about 5 minutes.
In a bowl, mix vinegar, water, sugar, dry mustard, and red pepper flakes until sugar has dissolved. Place the mustard greens into the onion and garlic mixture, and pour in the vinegar mixture. Stir to combine, cover the Dutch oven, and bring to a boil. Simmer the greens until the liquid has been absorbed, 5 to 10 minutes.
Mix in the white beans, allow to heat through, and season to taste with salt and black pepper.
