Spicy Southern Mustard Greens: My Family's Secret!

Ingredients

• 2 pounds (900g) fresh mustard greens, washed thoroughly and coarsely chopped
• 4 slices thick-cut bacon, diced
• 1 medium yellow onion, chopped (about 1 cup)
• 3 cloves garlic, minced
• 1/4 teaspoon red pepper flakes (or more to taste)
• 4 cups (950ml) chicken broth or vegetable broth
• 2 tablespoons apple cider vinegar
• 1 teaspoon granulated sugar
• Salt and freshly ground black pepper to taste
• 2 tablespoons olive oil or bacon fat (reserved from cooking the bacon)