Bring the water and 1 tablespoon of salt to a boil in a large saucepan. Slowly whisk in the cornmeal, then reduce the heat to low and cook for 15 to 20 minutes, stirring often, until the polenta thickens. Remove from the heat and stir in the milk, cheese, and butter. Season with salt and pepper, to taste.
Meanwhile, place a large skillet over medium-high heat. Add the bacon and cook for about 6 minutes, until golden and crispy. Using a slotted spoon, transfer the bacon to paper towels to drain.
Add the garlic and red pepper flakes to the pan with the bacon grease and cook for about 1 minute, until fragrant. Add the mustard greens, reduce the heat to low, and cook for 6 to 7 minutes, until the greens are tender and wilted. Stir in the cannellini beans and cook for another 2 minutes to warm the beans through.
Spoon the polenta into bowls and top with the mustard greens, cannellini beans, and bacon.